Remove size alias for length validation

scope_chain should not be mutated for other reflections

Fix `singleton_class?`

Removed ```The `size` helper is an alias for `length`.``` line. If you use this "nonexist" helper, you will get an error message like this: 

```
ArgumentError: Unknown validator: 'SizeValidator'
...
```

Maybe wanted to mean ```validates_size_of``` helper as an alias for ```validates_length_of``` helper.

Due to changes from http://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/39628 current `singleton_class?` implementation fails.
Changed based on reference from http://bugs.ruby-lang.org/issues/7609

`$SAFE = 4;` has been removed with Ruby 2.1

For background -
https://bugs.ruby-lang.org/issues/8468
Changset - https://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/41259/diff/test/ruby/test_thread.rb

Currently `scope_chain` uses same array for building different
`scope_chain` for different associations. During processing
these arrays are sometimes mutated and because of in-place
mutation the changed `scope_chain` impacts other reflections.

Fix is to dup the value before adding to the `scope_chain`.

Fixes #3882.

SQLite3 Adapter datatypes cleanup

Remove `default_primary_key_type`  and extract contains of `native_database_types` to a constant since they aren't conditional now in SQLite3Adapter. Makes it more like other adapters.

Merge pull request #9499 from dmitry/inverse_instance_should_not_be_reloaded_after_stale_state_change

Inversed instance should not be reloaded after stale state was changed

Conflicts:
	activerecord/CHANGELOG.md

Remove sqlite specific`supports_autoincrement?` which defaults to true

Fixes #12489

newline at end of structure.sql file

check at association reader that record is inverted and should not be reloaded because of stale was changed at target record

`Relation#count` doesn't use options anymore.

Merge pull request #12508 from jetthoughts/12415_generate_subqueries_for_relation_from_binding_params

Generate subquery for Relation passed as array condition for where

Conflicts:
	activerecord/CHANGELOG.md

Instead of executing 2 queries for fetching records filtered by array condition with Relation,
added generation of subquery to current query.

This behaviour will be consistent when passes Relation as hash condition to where

Closes: #12415

Fixed MRI dependent stuff in AR tests

Make missed association exception message more informative

Add target class name, which should have missed association on preload,
into exception message to simplify detecting problem part.

Stop accepting `options` for `Relation#average`, `Relation#minimum`, `Relation#maximum`, `Relation#calculate`, `perform_calculation`, `NullRelation#calculate` as they isn't used anymore.

Make logging to stdout work again with implicit `development` env

Fix wrong variable name used in the select_day method documentation

Keep code consistent with previous code blocks.

Change `map` to `map!` to save extra array creation on new array

Drop unused iterator var