Several changes were made in #21110 which I am strongly opposed to.
(this is what I get for going on vacation. :trollface:) No type should
be introduced into the generic `ActiveRecord::Type` namespace, and
*certainly* should not be registered into the registry unconstrained
unless it is supported by *all* adapters (which basically means that it
was specified in the ANSI SQL standard).

I do not think `# :nodoc:` ing the type is sufficient, as it still makes
the code of Rails itself very unclear as to what the role of that class
is. While I would argue that this shouldn't even be a super class, and
that MySql and PG's JSON types are only superficially duplicated (they
might look the same but will change for different reasons in the
future).

However, I don't feel strongly enough about it as a point of contention
(and the biggest cost of harming the blameability has already occured),
so I simply moved the superclass into a namespace where its role is
absolutely clear.

After this change, `attribute :foo, :json` will once again work with
MySQL and PG, but not with Sqlite3 or any third party adapters.

Unresolved questions
--------------------

The types that and adapter publishes (at least those are unique to that
adapter, and not adding additional behavior like `MysqlString` should
probably be part of the adapter's public API. Should we standardize the
namespace for these, and document them?

As of MySQL 5.7.8, MySQL supports a native JSON data type.

Example:

    create_table :json_data_type do |t|
      t.json :settings
    end

Passing `:from` and `:to` to `change_column_default` makes this command
reversible as user has defined its previous state.

So, instead of having the migration command as:

    change_column_default(:posts, :state, "draft")

They can write it as:

    change_column_default(:posts, :state, from: nil, to: "draft")

Related with #17370.

This line introduced by the commit fd398475 for using
`Arel::Visitors::BindVisitor`. Currently it is not used.

MySQL does not support partial index. And, the create index algorithm in
create table can not be specified.

Some databases like MySQL allow defining collation charset for specific
columns.

`change_column_null` is doc'ed only in
ActiveRecord::ConnectionAdapters::SchemaStatements, so it would make
sense to :nodoc: it elsewhere.

It behaves in the same way that the abstract adapter.

[ci skip]

to support MySQL 5.7.6 `optimizer_switch='derived_merge=on'`

Example:

    create_table :foos do |t|
      t.string :string_utf8_bin, charset: 'utf8', collation: 'utf8_bin'
      t.text   :text_ascii,      charset: 'ascii'
    end

MySQL unicode support is not only `utf8mb4`.
Then, The index length problem is not only `utf8mb4`.

http://dev.mysql.com/doc/refman/5.6/en/charset-unicode.html

    SELECT * FROM information_schema.character_sets WHERE maxlen > 3;
    +--------------------+----------------------+------------------+--------+
    | CHARACTER_SET_NAME | DEFAULT_COLLATE_NAME | DESCRIPTION      | MAXLEN |
    +--------------------+----------------------+------------------+--------+
    | utf8mb4            | utf8mb4_general_ci   | UTF-8 Unicode    |      4 |
    | utf16              | utf16_general_ci     | UTF-16 Unicode   |      4 |
    | utf16le            | utf16le_general_ci   | UTF-16LE Unicode |      4 |
    | utf32              | utf32_general_ci     | UTF-32 Unicode   |      4 |
    +--------------------+----------------------+------------------+--------+

Only `primary_key` should be extracted by d47357e2 in #19030, but
`new_coclumn_definition` was also extracted because #17631 is merged
previously, then #19030 is auto merged without conflicts.

This commit is for move back `new_column_definition` into
`TableDefinition`.

Example:

    create_table :foos, id: :primary_key, limit: 8 do |t|
    end

    # or

    create_table :foos, id: false do |t|
      t.column :id, limit: 8
    end

It is also necessary to format a time column like a datetime column.

This reverts commit 1502caef.

The test suite for the mysql adapter broke when this commit was used
with MySQL 5.6.

Conflicts:
	activerecord/CHANGELOG.md

We do this in the adapter classes specifically, so the types aren't
registered if we don't use that adapter. Constants under the PostgreSQL
namespace for example are never loaded if we're using mysql.

`precision: 0` was not dumped by f1a0fa9e.
However, `precision: 0` is valid value for PostgreSQL timestamps.

The various databases don't actually need significantly different
handling for this behavior, and they can achieve it without knowing
about the type of the object.

The old implementation was returning a string, which will cause problems
such as breaking TZ aware attributes, and making it impossible for the
adapters to supply their logic for time objects.

If timestamp column have the precision, it need to format according to
the precision of timestamp column.