Support derives on more types by allowing user to specify concrete types

*Credit to @kupiakos for this idea*

Our derives are not able to support all generic types. However, sometimes a user may not need a given trait to be implemented for every instantiation of type variables. In this case, we could permit them to specify concrete types for which they want zerocopy trait impls:

```rust
#[derive(IntoBytes)]
#[zerocopy(derive_for = Foo)]
#[repr(C)]
struct Foo(A, B);

// Emitted by derive:
unsafe impl IntoBytes for Foo { ... }
```