Implement sock_accept

With the addition of `sock_accept()` in `wasi-0.11.0`, wasmtime can now
implement basic networking for pre-opened sockets.

For Windows `AsHandle` was replaced with `AsRawHandleOrSocket` to cope
with the duality of Handles and Sockets.

For Unix a `wasi_cap_std_sync::net::Socket` enum was created to handle
the {Tcp,Unix}{Listener,Stream} more efficiently in
`WasiCtxBuilder::preopened_socket()`.

The addition of that many `WasiFile` implementors was mainly necessary,
because of the difference in the `num_ready_bytes()` function.

A known issue is Windows now busy polling on sockets, because except
for `stdin`, nothing is querying the status of windows handles/sockets.

Another know issue on Windows, is that there is no crate providing
support for `fcntl(fd, F_GETFL, 0)` on a socket.

Signed-off-by: Harald Hoyer <harald@profian.com>

crates/wasi-common/cap-std-sync/src/sched/unix.rs

@@ -90,6 +90,18 @@ fn wasi_file_fd(f: &dyn WasiFile) -> Option<BorrowedFd<'_>> {
     let a = f.as_any();
     if a.is::<crate::file::File>() {
         Some(a.downcast_ref::<crate::file::File>().unwrap().as_fd())
+    } else if a.is::<crate::net::TcpStream>() {
+        Some(a.downcast_ref::<crate::net::TcpStream>().unwrap().as_fd())
+    } else if a.is::<crate::net::TcpListener>() {
+        Some(a.downcast_ref::<crate::net::TcpListener>().unwrap().as_fd())
+    } else if a.is::<crate::net::UnixStream>() {
+        Some(a.downcast_ref::<crate::net::UnixStream>().unwrap().as_fd())
+    } else if a.is::<crate::net::UnixListener>() {
+        Some(
+            a.downcast_ref::<crate::net::UnixListener>()
+                .unwrap()
+                .as_fd(),
+        )
     } else if a.is::<crate::stdio::Stdin>() {
         Some(a.downcast_ref::<crate::stdio::Stdin>().unwrap().as_fd())
     } else if a.is::<crate::stdio::Stdout>() {