Merge patch series "`zerocopy` support"

Introduce support for `zerocopy` [1][2]:

    Fast, safe, compile error. Pick two.

    Zerocopy makes zero-cost memory manipulation effortless. We write
    `unsafe` so you don't have to.

It essentially provides derivable traits (e.g. `FromBytes`) and macros
(e.g. `transmute!`) for safely converting between byte sequences and
other types. Having such support allows us to remove some `unsafe` code.

It is among the most downloaded Rust crates (top #50 recent, top #100
all-time downloads; according to crates.io), and it is also used by the
Rust compiler itself.

The series starts with a few preparation commits, then the `zerocopy`
and `zerocopy-derive` crates are added. Finally, an example patch using
it is on top, removing one `unsafe impl`.

I had to adapt the crates slightly (just +2/-3 lines), but both patches
could potentially be provided upstream eventually. Please see the
commits for details.

In total, it is about ~39k lines added, ~32k without counting `benches/`
which are just for documentation purposes.

See the cover letter for `syn` for some more details about depending on
third-party crates in commit 54e3eae85562 ("Merge patch series "`syn`
support"").

The codegen of an isolated example function similar to the patch on top
is essentially identical. It also turns out that (for that particular
case) `zerocopy`'s version, even under `debug-assertions` enabled, has
no remaining panics, unlike a few in the current code (because the
compiler can prove the remaining `ub_checks` statically).

So their "fast, safe" does indeed check out -- at least in that case.

P.S. This version of `zerocopy` has already the unstable `Ptr{,Inner}`
types -- to play with them, please use:

    make ... KRUSTFLAGS=--cfg=zerocopy_unstable_ptr

Link: https://github.com/google/zerocopy [1]
Link: https://docs.rs/zerocopy [2]
Link: https://patch.msgid.link/20260608141439.182634-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>