Fix `COUNT(DISTINCT ...)` with `ORDER BY` and `LIMIT`

Follow up of #24203.

Since b644964b `ActiveRecord::Relation` includes `Enumerable` so it is
enough to call `super` simply.

When a grouped calculation contains a having clause that references a
selected value, we need to include that selected value in the query.

Postgres doesn't support referencing a selected value in a having
clause, but other databases do; we can skip the test on the pg adapter
but run it for the others.

This was fixed before in 9a298a16, but
the test coverage was lost in 5a05207d.
The fix regressed in 6311975f and was
removed in 97d46c17.

`select_values` is a local variable defined at previous line.
`select_values += select_values` is totally useless.

`DISTINCT` clause is applied inside aggregate function by
`operation_over_aggregate_column` if needed. Unneeded outside aggregate
function.

```ruby
  # Before
  author.unique_categorized_posts.count
  # => SELECT DISTINCT COUNT(DISTINCT "posts"."id") FROM "posts" INNER JOIN "categorizations" ON "posts"."id" = "categorizations"."post_id" WHERE "categorizations"."author_id" = ?  [["author_id", 2]]

  # After
  author.unique_categorized_posts.count
  # => SELECT COUNT(DISTINCT "posts"."id") FROM "posts" INNER JOIN "categorizations" ON "posts"."id" = "categorizations"."post_id" WHERE "categorizations"."author_id" = ?  [["author_id", 2]]
```

Closes #27615

This reverts commit 28977f1f.

If select clause is specified and last column has a column alias,
additional column alias causes a statement invalid.

Add test coverage for counting a single column with NULL values.

Fixes #27676, #27682, and #27705.

Actually, private methods cannot be called with `self.`, so it's not just redundant, it's a bad habit in Ruby

If `limit_value` is presented, records fetching order is very important
for performance. Should not unscope the order in the case.

All indentation was normalized by rubocop auto-correct at 80e66cc4.
But comments was still kept absolute position. This commit aligns
comments with method definitions for consistency.

`column_for` was changed to `type_for` to return `type` object at
36bd52b4. But variable name is still `column`. It is very confusing.
Rename variable name `column` to `type` for readability.

For reduce instantiating `Type::Value`.

The current behaviour of checking if there is a LEFT OUTER JOIN arel
node to detect if we are doing eager_loading is wrong. This problem
wasn't frequent before as only some pretty specific cases would add
a LEFT OUTER JOIN arel node. However, the recent new feature
left_outer_joins also add this node and made this problem happen
frequently.

Since in the perform_calculation function, we don't have access to
eager_loading information, I had to extract the logic for the distinct
out to the calculate method.

As I was in the file for left_outer_join tests, I fixed a few that had
bugs and I replaced some that were really weak with something that
will catch more issues.

In relation tests, the first test I changed would have failed if it
had validated the hash returned by count instead of just checking how
many pairs were in it. This is because this merge of join currently
transforms the join node into an outer join node, which then made
count do a distinct. So before this change, the return was
{1=>1, 4=>1, 5=>1}.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

When `group` is used in combination with any calculation method, the
resulting hash uses the grouping expression as the key. Currently we're
incorrectly always favoring the type reported by the query, instead of
the type known by the class. This causes differing behavior depending on
whether the adaptor actually gives proper types with the query or not.
After this change, the behavior will be the same on all adaptors -- we
see if we know the type from the class, fall back to the type from the
query, and finally fall back to the identity type.

Fixes #25595

Ruby 2.4 unifies Fixnum and Bignum into Integer: https://bugs.ruby-lang.org/issues/12005

* Forward compat with new unified Integer class in Ruby 2.4+.
* Backward compat with separate Fixnum/Bignum in Ruby 2.2 & 2.3.
* Drops needless Fixnum distinction in docs, preferring Integer.

This still isn't as separated as I'd like, but it at least moves most of
the burden of alias mapping in one place.

* When tried to use `Company#accounts` test/models/company.rb I got:

```
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column:
accounts.company_id: SELECT COUNT(*) AS count_all, "companies"."firm_id"
AS companies_firm_id FROM "companies" INNER JOIN "accounts" ON
"accounts"."company_id" = "companies"."id" GROUP BY "companies"."firm_id"
```

* The refactor on Calculations class was just to simplify the code

Closes #21922

Let `Book(id, author_id)`, `Photo(id, book_id, author_id)` and `Author(id)`

Running `Book.group(:author_id).joins(:photos).count` will produce:

* Rails 4.2 - conflicts `author_id` in both projection and group by:
```sql
SELECT COUNT(*) AS count_all, author_id AS author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY author_id
```

* Master (9d02a25d) - conflicts `author_id` only in projection:
```sql
SELECT COUNT(*) AS count_all, author_id AS author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY "books"."author_id"
```

* With this fix:
```sql
SELECT COUNT(*) AS count_all, "books"."author_id" AS books_author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY "books"."author_id"
```

Column names inserted via `group` have to be qualified with table name.

The focus of this change is to make the API more accessible.
References to method and classes should be linked to make it easy to
navigate around.

This patch makes exzessiv use of `rdoc-ref:` to provide more readable
docs. This makes it possible to document `ActiveRecord::Base#save` even
though the method is within a separate module
`ActiveRecord::Persistence`. The goal here is to bring the API closer to
the actual code that you would write.

This commit only deals with Active Record. The other gems will be
updated accordingly but in different commits. The pass through Active
Record is not completely finished yet. A follow up commit will change
the spots I haven't yet had the time to update.

/cc @fxn

This removes the following warning.
```
activerecord/lib/active_record/relation/calculations.rb:74: warning: `&' interpreted as argument prefix
```

Arguments of `#sum` does not match with other shortcuts methods
(count, average, minimum, and maximum).
This commit fix these two points:

* call `super` with only block arguments
  First argument of `super` method, `Enumerable#sum`, is `identity`
  and first argument of `AR::Calculations#sum` is `column_name`.
  `Enumerable#sum` does not expect `column_name` to be passed.
* Change first argument of `sum` from array arguemnt to single
  argument to match other shortcuts methods. When `sum` accept
  array arguemnt, user can pass multi arguments and an exception is
  raised from `calculate`.

as #uniq will be removed from Rails 5.0 as per the Active Support
exception raised:

ActiveSupport::DeprecationException: DEPRECATION WARNING: uniq is
deprecated and will be removed from Rails 5.0 (use distinct instead).

It allows a query like `User.select(:name).count` to be written
using Arel as `User.select(User.arel_table[:name]).count`.

It exposes the calculations API to accept Arel nodes:
`User.count(User.arel_table[:name])`, `User.sum(User.arel_table[:id])`,
`Account.average(Account.arel_table[:credit_limit])`,
`Account.maximum(Account.arel_table[:credit_limit])` and
`Account.minimum(Account.arel_table[:credit_limit])`.

This is to show users that they can chain `.uniq` and `.pluck` to get
the `DISTINCT column` result. They don't have to do `DISTINCT column`
themselves.

After discussing, we've decided it makes more sense to include it. We're
already forwarding every conflicting method to `to_a`, and there's no
conflation of concerns. `Enumerable` has no mutating methods, and it
just allows us to simplify the code. No existing methods will have a
change in behavior. Un-overridden Enumerable methods will simply
delegate to `each`.

[Sean Griffin & bogdan]

This matches our behavior in other cases where useful enumerable methods
might have a different definition in `Relation`. Wanting to actually
enumerate over the records in this case is completely reasonable, and
wanting `.sum` is reasonable for the same reason it is on `Enumerable`
in the first place.

This allows easier integration with ActiveRecord, such that
AR#pluck will now use Enumerable#pluck if the relation is loaded,
without needing to hit the database.

Documentation is giving an example that can be replaced by a more dry command.
Give a hint that ids can be used instead of pluck(:id).

It is redundant with tests in `eager_loading?`, but for the difference
between `includes_values.present?` and `includes_values.any?`, which
is a difference without a distinction because `false` has no meaning
for `includes`.

[ci skip]