Lightweight generics support

[Manishearth]

In theory, Diplomat could support generics, provided each generic can only pull from a closed set of types.

Something like this:

#[diplomat::generictrait]
pub trait SomeTrait {...}

impl SomeTrait for crate::module::ffi::SomeOpaqueType;
impl SomeTrait for crate::module::ffi::SomeOtherOpaqueType;
impl SomeTrait for u8;

pub struct GenericOpaque<T: SomeTrait>(RustType<T>);

impl GenericOpaque {
   fn create(t: &T) -> Box<Self> {...}
}

This would generate C ABI functions create_SomeOpaqueType, create_SomeOpaqueType, etc. In C++ this would produce a specialized, templated GenericOpaque<T> type. In JS it would be a single type with matchers on its methods.

If you want to use traits from other modules you'd need to define some type of #[diplomat::trait_use = src/traits.rs] thing so the macro can use them (see #752 for a similar design issue).

This is likely to be quite complicated, needing major changes to the AST, HIR, and individual backends.