Support parameterized types in the DSLX-to-C++ transpiler

(Note that I'm currently working on said transpiler)

On the first pass, we won't support instantiating parameterized types in the transpiler, because A) my use cases don't need it, and B) its hard, but it would be good to have. It'd require some thought as to what we'd be targeting and how we'd want to expose this to a user - are we targeting the JIT? Then we _could_ emit fully instantiated parametrics. Would we be targeting arbitrary usages? Then we'd want...templatized C++ types? Or regular C++ types that could be size-adjusted dynamically?

Not a trivial task.