Support querying on multiple locales at once

[shioyama]

This is currently supported (although not tested) by all backends except KeyValue and Table. Should be straightforward to support universally.

The format looks like this:

Post.i18n(locale: :en) do |en|
  Post.i18n(locale: :ja) do |ja|
    en.title.eq("Title").and(ja.title.eq("タイトル"))
  end
end

This queries for a post with an English title Title and a Japanese title タイトル, but obviously you could build much more complex logic.

Here is the resulting SQL for a Jsonb backend:

SELECT "posts".* FROM "posts"
WHERE ("posts"."title" -> 'en') = '"foo"' AND ("posts"."title" -> 'ja') = '"bar"'

The problem with KeyValue and Table backends is that their respective joins do not alias the table name with an alias that includes the locale.