When requesting columns names from database adapters AR:Result
would dup/freeze column names, this prefers using fstrings which
cuts down on repeat allocations

Attributes that are retained keep these fstrings around for the long
term

Note, this has the highest impact on "short" result sets, eg: Topic.first where you can void allocating the number of columns * String.

Since #32028, Rails 6 requires Ruby 2.3+.
No longer needed workaround for Ruby 2.2 "private attribute?" warning.

It is useless since 90c8be76.

This reverts commit 3420a145, reversing
changes made to afb66a5a.

`display_deprecation_warning_for_false_terminator` was removed since
3a25cdca.

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.

Some case expressions remain, need to think about those ones.

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

Do not specal case inspecting associated arrays of over 10 elements, preventing infinite looping in some cases.

The `#[]` method *used to be* an alias of `#read_attribute`, but since Rails 4
(10f6f90d), it will raise an exception for
missing attributes. Saying that it is an alias is confusing.

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.

`has_attribute?` method to check wether a given attribute has been defined.

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

The thread_safe gem is being deprecated and all its code has been merged
into the concurrent-ruby gem. The new class, Concurrent::Map, is exactly
the same as its predecessor except for fixes to two bugs discovered
during the merge.