Support more types

[harpocrates]

I think we will want:

Haskell	Rust argument	Status
StorableArray Int a	&mut [a']	see #12
Vector a	Vec<a'>
StablePtr a	*const a'
()	()	Done
(a,b)	(a',b')	Done
(a,b,c)	(a',b',c')	Done
Storable data	corresponding #[repr(C)] struct	Done
ByteString	&[u8]	see #12
Maybe a	Option<a'>	Done
Either e a	Result<a',e'>	Done

I'm still undecided on what to do around strings. I also feel like we could make good use of box somewhere here...

[ghost]

This will be really cool when it works. Will using tree/graph-like types be feasible?
(I think that's ADTs?)

[harpocrates]

@22459 Yeah, I'd like to eventually have Haskell ADTs mapping to Rust enums. Right now, I only support passing things that have the same memory-representation, so I'd have to add support for some extra processing on both the Haskell and Rust side.

Really, I just need to find more time to work on this.

[ghost]

Right. Good luck!