This helper no longer makes sense as a separate method. Instead I'll
just have `deserialize` call `cast` by default. This led to a random
infinite loop in the `JSON` pg type, when it called `super` from
`deserialize`. Not really a great way to fix that other than not calling
super, or continuing to have the separate method, which makes the public
API differ from what we say it is.

Adding `# :nodoc:` to the parent `class` / `module` is not going
to ignore nested classes or modules.

There is a modifier `# :nodoc: all` but sadly the containing class
or module will continue to be in the docs.

/cc @sgrif

This inlines casting for the most obvious types. The rest will
follow eventually. I need to put some tests in place, to make sure
that the inlining is not causing regressions.

/cc @sgrif

As we promote these classes to first class concepts, these classes are
starting to gain enough behavior to warrant being moved into their own
files. Many of them will become quite large as we move additional
behavior to the type objects.