rust: zerocopy: import crate

rust: zerocopy: import crate

This is a subset of the Rust `zerocopy` crate, version v0.8.50 (released
2026-05-31), licensed under "BSD-2-Clause OR Apache-2.0 OR MIT", from:

    https://github.com/google/zerocopy/tree/v0.8.50

The files are copied as-is, with no modifications whatsoever (not even
adding the SPDX identifiers).

The `benches` folder is added (i.e. not just `src` like in other cases)
since the files there are included in the rendered documentation,
as well as the `rustdoc` CSS style file that is needed to make those
visually more understandable.

For copyright details, please see:

    https://github.com/google/zerocopy/blob/v0.8.50/README.md?plain=1
    https://github.com/google/zerocopy/blob/v0.8.50/LICENSE-BSD
    https://github.com/google/zerocopy/blob/v0.8.50/LICENSE-APACHE
    https://github.com/google/zerocopy/blob/v0.8.50/LICENSE-MIT

The next two patches modify these files as needed for use within the
kernel. This patch split allows reviewers to double-check the import
and to clearly see the differences introduced.

The following script may be used to verify the contents:

    for path in $(cd rust/zerocopy/ && find . -type f); do
        curl --silent --show-error --location \
            https://github.com/google/zerocopy/raw/v0.8.50/$path \
            | diff --unified rust/zerocopy/$path - && echo $path: OK
    done

Cc: Joshua Liebow-Feeser <joshlf@google.com>
Cc: Jack Wrenn <jswrenn@google.com>
Link: https://patch.msgid.link/20260608141439.182634-9-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>