Support ADTs

[harpocrates]

Extend contexts support passing Haskell ADTs directly into Rust enums. The core insights are that

• Storable lets us reorganize what exactly gets passed through the FFI. On the Rust side, we need to make that explicit by having an intermediate #[repr(C)] type and some conversions to and from that type.
• These sorts of conversion shenanigans only work when we are passing the actual values through the FFI - functions and pointers won't work for this (another face of the same observation: the Storable instances for Ptr a and FunPtr a do not impose superclass Storable constraints)

When merged, this PR will go a long way to helping on #1.

• Extend contexts to support intermediate Rust types.
• Generate #[repr(C)] unions/structs corresponding to the derived Storable instances for ADTs
• Generate enums corresponding to the ADTs
• Generate Into/From trait impls for converting between the #[repr(C)] union/struct form and the enum form
• Wrap all of this into a simple TH function
• Update all of the documentation

[harpocrates]

Couple points now that I've got an MVP:

• every step of the codegen needs to be manually overridable
• I need to generate an extra From impl for just the generic parameters (impl<T, T1: Into<T>> From<Option<T>> for Option<T1>)

[harpocrates]

Couple points now that I've got an MVP:

• every step of the codegen needs to be manually overridable
• I should support top-level reference types (by unsafely converting a pointer from a reference argument into a reference) - but only for types that require no conversion
• I need to solve the issue around a missing (and impossible) From impl like impl<T, T1: Into<T>> From<MyStruct<T>> for MyStruct<T1>.
• Better error messages (thread some Writer through?)