we want to go through methods to access `env` because in the future that
ivar may not be available, or may be calculated lazily

now the parameter filter doesn't need to know about the env hash in
these two methds.

Now the Headers internals don't depend on the env hash.

This prevents external mutations from impacting the internals of the
request or the Header object.

this reduces the API footprint for the env hash so that we can be more
flexible when changing API in the future

This allows us to avoid calling `env_name` twice.

duping the request will dup it's underlying env hash.

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?

`ActiveSupport::Dependencies.constantize(const_name)` calls
`Reference.new` which is defined as
`ActiveSupport::Dependencies.constantize(const_name)` meaning this call
is already cached and we're doing caching that isn't necessary.

I'm going to change the constructor, so it's easier to do it in one
place.

Remove duplicity in tests

Small fixes [ci skip]

Fix Punctuation in `AutosaveAssociation` RDoc [ci skip]

This way we can make the Route object a read-only data structure.

[ci skip]

Removed mocha from ActionMailer

File renaming should be the last operation of an atomic write

fix path of annotations.rake [ci skip]

Closes #21304.

While we can validate uniqueness for record without primary key on
creation, there is no way to exclude the current record when
updating. (The update itself will need a primary key to work correctly).

Correct cache store superclass in comment [ci skip]

Cleaned up generators tests using internal assertion helper

Migrate to Sprockets 4.

Add code of conduct info to README.md and to contributing guide

Adds a code of conduct

Added docs for TableDefinition #coloumns & #remove_column [ci skip]