Allocator doesn't seem to be implemented for Arc<A>

[lachlansneff]

This has obvious usecases.

impl<A: Allocator + ?Sized, B: Allocator> Allocator for Arc<A, B> {
    ...
}

[TimDiekmann]

This was already proposed in #54, however the Rc and Arc part were removed as the allocator had to be mutable. I changed back the proposal to include it again. Adding a parameter to Arc is tracked in #7.