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/src/error.rs

@@ -30,6 +30,9 @@ pub use anyhow::{Context, Error};
 /// the crate. Not all values are represented presently.
 #[derive(Debug, thiserror::Error)]
 pub enum ErrorKind {
+    /// Errno::WouldBlk: Would block
+    #[error("WouldBlk: Would block")]
+    WouldBlk,
     /// Errno::Noent: No such file or directory
     #[error("Noent: No such file or directory")]
     Noent,