Consider matching box types to ForeignPtr

[harpocrates]

Consider

newtype Box t = Box { unBox :: ForeignPtr t }

It might be interesting to match Box<T> types to Box t. The useful part would be being able to associated a Haskell finalizer with Boxs we get back from Rust (the finalize would just look like [rust| () { $(x: Box<..>).drop() } |]).

[harpocrates]

The Box type here is what I want if -XLinearTypes goes through.