As part of this, I've moved a couple of methods for checking for
null/undefined from UnionType to Type, and filled in more of their
steps.
This now detects more, and so causes us to hit a `TODO()` which is too
big for me to go after right now, so I've replaced that assertion with
a log message.
Track the kind of Type it is, and use that to provide some convenient
`is_foo()` / `as_foo()` methods. While I was at it, made these all
classes instead of structs and made their data private.
IDL function overload resolution requires knowing each IDL function's
parameters and their types at runtime. The simplest way to do that is
just to make the types the generator uses available to the runtime.
Parsing has moved to LibIDL, but code generation has not, since that is
very specific to WrapperGenerator.
