Top-level reference types

[harpocrates]

References at the top level should be possible. For example:

main = do
  let s: S = {- some complex deeply nested struct (but not enum!) -}
  [rustIO| () { println!("{:#?}", (&s: &S)) } |]

Under the hood, this should turn into a Ptr S which gets unsafely turned into a reference on the Rust side.

[harpocrates]

Types that we will want to have in this category:ByteString, ByteArray, ByteArray#, MutableByteArray, MutableByteArray#, StorableVector.