Extensions

Every OpCode should add:

• It's own separate crate (e.g. io-uring-epoll) and it's own type (e.g. EpollCtl).
• Impl OpCode and OpCompletion from the io-uring-opcode crate.
• Feature-gated "extension" trait within io-uring-opcode similar to OpExtEpollCtl.
• Feature-gated UringBearer::Completion variant, e.g. Completion::EpollCtl within UringBearer.
• Explicit API to push the OpCode as submission, e.g. UringBearer::push_epoll_ctl