Decouple WASIp2 sockets from WasiFd
This is a follow up to #129638, decoupling WASIp2's socket implementation from WASIp1's `WasiFd` as discussed with `@alexcrichton.`
Quite a few trait implementations in `std::os::fd` rely on the fact that there is an additional layer of abstraction between `Socket` and `OwnedFd`. I thus had to add a thin `WasiSocket` wrapper struct that just "forwards" to `OwnedFd`. Alternatively, I could have added a lot of conditional compilation to `std::os::fd`, which feels even worse.
Since `WasiFd::sock_accept` is no longer accessible from `TcpListener` and since WASIp2 has proper support for accepting sockets through `Socket::accept`, the `std::os::wasi::net` module has been removed from WASIp2, which only contains a single `TcpListenerExt` trait with a `sock_accept` method as well as an implementation for `TcpListener`. Let me know if this is an acceptable solution.
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