bearer: Migrate OpCodes to trait Impls

Remaining OpCodes in bearer to migrate away to ops/ to impl OpCode trait
- [ ] accept.rs - `Accept`
- [ ] buffers.rs - `ProvideBuffers`
- [ ] futex.rs - `FutexWait`
- [ ] recv.rs - `Recv`, `RecvMulti`
- [ ] send_zc.rs - `SendZc`

Question is where do the associated buffers and futexes (and handling of those) live I guess beyond the opcodes

Related:
- #42 
- #34 
- #21 