Support arrays of GC-allocated objects

[Techcable]

This is related to #15 because [T] is unsized. Ideally, we would be able to have Gc<'gc, [i32]> without any indirection through a Box<[T]>.

The big problem here is that the simple collector expects Gc to have a statically-known size......

This is very important to actually properly implement any real-world use. In theory, arrays could just wrap Vec right now but that's not very elegant.