Migrate to foreign_type! macro

[kvark]

See https://docs.rs/foreign-types/0.4.0/foreign_types/
What's not clear to me is how this works with a parent type semantics: current macro in metal-rs supports it and uses it, but I don't see it in the foreign_types crate. cc @sfackler

[sfackler]

What do you mean by parent type semantics? Upcasting? You'd probably end up making a separate trait and macro that wraps foreign_types!.

[kvark]

@sfackler I mean this -

metal-rs/src/lib.rs

Line 66 in 1b0c776

type ParentType = $parent_ref:ident;

On a second look, it's just one impl Deref away from what you have. So we can just add it ourselves.