rust: prelude: add `zerocopy{,_derive}::IntoBytes`

rust: prelude: add `zerocopy{,_derive}::IntoBytes`

In order to easily use `IntoBytes`, add it to the prelude.

This adds both the trait (`zerocopy::IntoBytes`) as well as the derive
macro (`zerocopy_derive::IntoBytes`).

[ This patch will simplify using the feature in several trees next cycle.

  Gary writes:

    This is wanted because I want to convert the upcoming I/O projection
    series to use `zerocopy` traits rather than keep using transmute
    module.

    It is most helpful for derives in doc tests; I do not want to
    explicitly use `#[derive(zerocopy_derive::IntoBytes)]` in the
    doc tests.

  For reference, the upcoming I/O projection series is at:

    https://lore.kernel.org/rust-for-linux/20260611-io_projection-v4-0-1f7224b02dcb@garyguo.net/

      - Miguel ]

Signed-off-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260611134802.2052296-1-gary@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>