rust: Import upstream `alloc::vec::set_len_on_drop` module

rust: Import upstream `alloc::vec::set_len_on_drop` module

This is a subset of the Rust standard library `alloc` crate,
version 1.62.0, licensed under "Apache-2.0 OR MIT", from:

    https://github.com/rust-lang/rust/tree/1.62.0/library/alloc/src

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

For copyright details, please see:

    https://github.com/rust-lang/rust/blob/1.62.0/COPYRIGHT

Signed-off-by: Asahi Lina <lina@asahilina.net>
Link: https://lore.kernel.org/r/20230224-rust-vec-v1-1-733b5b5a57c5@asahilina.net
[ Import version 1.62.0 instead, to match the one in mainline. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>