Otherwise, we end up propagating those dependencies into targets that
link against that library, which creates unnecessary link-time
dependencies.
Also included are changes to readd now missing dependencies to tools
that actually need them.
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.
