[zircon][rust] Add Rust SpscBuffer implementation

Ports the transactional, single-producer, single-consumer (SPSC) ring
buffer library from C++ to Rust. This implementation matches the C++
interface design, while leveraging Rust safety features to avoid unsafe
memory copying in `write` and `read` operations. It is meant to
interoperate transparently with the C++ version.

Bug: 507423574
Bug: 517305548
Change-Id: I3b28cc68fde8079afec4b82cdfcc8b4b0df0d9e9
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1650574
Fuchsia-Auto-Submit: Abdulla Kamar <abdulla@google.com>
Reviewed-by: Adam Barth <abarth@google.com>
Commit-Queue: Abdulla Kamar <abdulla@google.com>
Reviewed-by: Adrian Danis <adanis@google.com>