rust: list: add iterators

Rust Binder has lists containing stuff such as all contexts or all
processes, and sometimes needs to iterate over them. This patch enables
Rust Binder to do that using a normal for loop.

The iterator returns the ArcBorrow type, so it is possible to grab a
refcount to values while iterating.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240814-linked-list-v5-7-f5f5e8075da0@google.com


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>