Idea: `AsBytes` convenience macro and trait for `IntoBytes + Immutable`

It would be ergonomic to have an `AsBytes` derive macro to derive both `IntoBytes` and `Immutable`, as well as a convenience blanket-implemented trait `AsBytes` for all `T: ?Sized + Immutable + IntoBytes`. This would [not be](https://docs.rs/zerocopy/latest/zerocopy/byte_slice/trait.ByteSliceMut.html#impl-ByteSliceMut-for-B) the first blanket-implemented trait in zerocopy 0.8.

This'd shorten the common `#[derive(FromBytes, IntoBytes, Immutable)]` and `T: Immutable + IntoBytes` into `#[derive(AsBytes, FromBytes)]` and `T: AsBytes`.

It'd also help migrations from 0.7 to some degree, even if the details of the names are changing.