compiler/rust: Move VecPair from NAK to shared compiler crate

Move the VecPair<A, B> data structure from NAK's ir.rs to the shared
compiler Rust crate so it can be reused by other backends.

The fields are private, and NAK's ir.rs (now in a different crate)
needs to read and mutate the inner Vecs. Add a_as_slice(..),
a_as_mut_slice(..), b_as_slice(..) and b_as_mut_slice(..), and update
NAK's SrcsAsSlice and DstsAsSlice impls to call them. Returning slices
keeps callers from changing the length of one side without the other,
which is what VecPair is built to prevent.

Signed-off-by: Christian Gmeiner <cgmeiner@igalia.com>
Reviewed-by: Mel Henning <mhenning@darkrefraction.com>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41435>