Extend contexts to prepare for deriving via TH Rust structs/unions and enums
to match ADTs. All old examples still work.

It will need to be reused for deriving Rust types.

The Rust codegen that remains is the `impl`s.

Added support for generating the right pairs of `From<...> for ...`
impl's for the ADT case. There are still a handful of small TODOs
before this is testable though.

I've manually verified that the generated code compiles.

The one major hiccup: Haskell's FFI supports a very limited number of types.
Consequently, I'm shoving any other type into a pointer (alloca/with) and
passing the pointer instead. For complex return values, that means passing
in a pointer and assigning to that pointer.

Added a context for unboxed types too.

Replaced uses of 'Into' with a custom 'MarshalInto'. Obviously
this isn't great, but it does mean I can make the struct impl's I
want (without conflicting with some more general impl).