Reorganize ActiveRecord tasks [Arun Agrawal & Abd ar-Rahman Hamidi]
Use destructured arguments when looping through pairs

Minor refactoring of looping behavior for aggregation
Make `:index` in migrations work with all column types
early return on delete and destroy methods

When delete or destroy is called on all records nothing
is deleted or destroyed. Intead of running through the code and still
not deleteing anything, we should early return
pg guide, move introductory sentences into main content. [ci skip]

This is a reaction to https://github.com/rails/rails/commit/7ca75f3789e316b6f3dc1f8000ec4f85c8ba32ea#commitcomment-6303828
Merge pull request #14962 from arunagw/aa-fix-rake-activerecord

Reorganize ActiveRecord tasks [Arun Agrawal & Abd ar-Rahman Hamidi]
pg, add missing nodocs for extracted modules.
pg, re-introduce `PostgreSQL::Utils` to unify schema/table extraction.

Partial revert of c0bfc3f412834ffe8327a15ae3a46602cc28e425
Merge pull request #15160 from sgrif/sg-destructured-each

Use destructured arguments when looping through pairs
Merge pull request #14126 from schuetzm/index_option_for_column

Make `:index` in migrations work with all column types
Merge pull request #15168 from eileencodes/return-early-on-delete-and-destroy-methods

Return early on delete and destroy methods
test, inline `DeveloperWithAggregate`, which is used by a single test.
Delegate `Column#type` to the injected type object

The decision to wrap type registrations in a proc was made for two
reasons.

1. Some cases need to make an additional decision based on the type
  (e.g. a `Decimal` with a 0 scale)
2. Aliased types are automatically updated if they type they point to is
  updated later. If a user or another adapter decides to change the
  object used for `decimal` columns, `numeric`, and `number` will
  automatically point to the new type, without having to track what
  types are aliased explicitly.

Everything else here should be pretty straightforward. PostgreSQL ranges
had to change slightly, since the `simplified_type` method is gone.
Merge pull request #15158 from sgrif/sg-delegate-type

Delegate `Column#type` to the injected type object