Per [this discussion][arel-discussion] on the discourse forum, this is
an addition to Arel for supporting `@>` (contains) and `&&` (overlaps)
operators in PostgreSQL. They are useful for GIN-indexed data such as a
`jsonb` or array column.

  [arel-discussion]: https://discuss.rubyonrails.org/t/what-has-happened-to-arel/74383/51

When using PostgreSQL, it's useful to be able to specify NULLS FIRST and NULLS LAST on ordered columns. With this commit you can do that now, as in:

```ruby
User.arel_table[:first_name].desc.nulls_last
```

We sometimes say "✂️ newline after `private`" in a code review (e.g.
https://github.com/rails/rails/pull/18546#discussion_r23188776,
https://github.com/rails/rails/pull/34832#discussion_r244847195).

Now `Layout/EmptyLinesAroundAccessModifier` cop have new enforced style
`EnforcedStyle: only_before` (https://github.com/rubocop-hq/rubocop/pull/7059).

That cop and enforced style will reduce the our code review cost.

roflscaling (using frozen string constants instead of literal
strings) was added in 2012, before frozen string literals were
added in Ruby 2.3.  Now that Rails no longer supports Ruby <2.3,
and all of these files use frozen string literals, there is
no reason to keep the roflscaling.

This does not delete or deprecate the related constants. Such
a change can be made in a later commit.

* Arel: Implemented DB-aware NULL-safe comparison

* Fixed where clause inversion for NULL-safe comparison

* Renaming "null_safe_eq" to "is_not_distinct_from", "null_safe_not_eq" to "is_distinct_from"

[Dmytro Shteflyuk + Rafael Mendonça França]