Chore: remove `MakeSendMarker` in favor of direct methods on the underlying types

[vn971]

Currently, async-usercalls uses an artificial MakeSendMarker trait to specify type boundaries for ByteBuffer, User<ByteBuffer>, User<[u8]> (see async-usercalls/src/utils.rs).

Instead, we could implement some of the underlying methods in those 3 types directly. This requires "upstreaming", so we'll first have to wait for async-usercalls changes to be accepted in the rust-lang repository.

[raoulstrackx]

Started on a branch for the compiler

[raoulstrackx]

Submitted a PR