Generalize the Write impl for `Vec<u8>` to `Vec<u8, A>`

I'd like to generalize the [`Write` impl for `Vec<u8>`](https://doc.rust-lang.org/std/vec/struct.Vec.html#impl-Write) to `Vec<u8, A>`. Is there any reason for not doing this? I suspect that this impl might simply have been forgotten because it lives in the `io` module rather than by the other trait impls for `Vec`?