feat: add support for splitting TcpStream and UnixStream

[teskje]

This PR adds the missing TcpStream::{into_}split and UnixStream::{into_}split APIs, as a step towards tokio parity.

UnixStream is currently just a mock type, so the new read/write half types are as well.

TcpStream has full support. Unfortunately, to support the borrowed ReadHalf/WriteHalf types, I had to make TcpStream interior-mutable by wrapping some of its fields into Mutexs. The resulting need for locking is a bit annoying. Lmk if anyone has a better idea!

[BugenZhao]

Unfortunately, to support the borrowed ReadHalf/WriteHalf types, I had to make TcpStream interior-mutable by wrapping some of its fields into Mutexs. The resulting need for locking is a bit annoying.

Using a spin lock here looks acceptable to me, as there appears to be no contention and it's just used for working around the borrow checker.

However, I'm still wondering if it's inevitable. Ideally the reader and the sender operates on different field of the TcpStream, thus it's feasible to split the mutable reference into a pair. Would you share more information on the difficulty you've encountered when attempting to implement it?