Add basic TCP socket APIs.

This is based on @npmccullum's [wasi-snapshot-preview2 draft] which is
in turn based on the [wasi-sockets proposal], though for simplify for
now it omits UDP sockets and some other features.

It's also based on the [pseudo-streams PR]; so that should proceed
first before this.

[draft wasi-snapshot-preview2]: https://github.com/npmccallum/wasi-snapshot-preview2
[wasi-sockets proposal]: https://github.com/WebAssembly/wasi-sockets
[pseudo-streams PR]: #29

host/src/tcp.rs

@@ -1,17 +1,136 @@
+#![allow(unused_variables)]
+
 use crate::{
-    wasi_tcp::{self, BytesResult, Socket, WasiTcp},
-    HostResult, WasiCtx,
+    wasi_tcp::{
+        self, ConnectionFlags, Errno, IpSocketAddress, ListenerFlags, Network, Size, TcpConnection,
+        TcpListener, WasiTcp,
+    },
+    HostResult, WasiCtx, WasiStream,
 };
 
 #[async_trait::async_trait]
 impl WasiTcp for WasiCtx {
-    async fn bytes_readable(&mut self, socket: Socket) -> HostResult<BytesResult, wasi_tcp::Error> {
+    async fn listen(
+        &mut self,
+        network: Network,
+        address: IpSocketAddress,
+        backlog: Option<Size>,
+        flags: ListenerFlags,
+    ) -> HostResult<TcpListener, Errno> {
+        todo!()
+    }
+
+    async fn accept(
+        &mut self,
+        listener: TcpListener,
+        flags: ConnectionFlags,
+    ) -> HostResult<(TcpConnection, IpSocketAddress), Errno> {
+        todo!()
+    }
+
+    async fn connect(
+        &mut self,
+        network: Network,
+        local_address: IpSocketAddress,
+        remote_address: IpSocketAddress,
+        flags: ConnectionFlags,
+    ) -> HostResult<TcpConnection, Errno> {
+        todo!()
+    }
+
+    async fn send(&mut self, connection: TcpConnection, bytes: Vec<u8>) -> HostResult<Size, Errno> {
+        todo!()
+    }
+
+    async fn receive(
+        &mut self,
+        connection: TcpConnection,
+        length: Size,
+    ) -> HostResult<(Vec<u8>, bool), Errno> {
+        todo!()
+    }
+
+    async fn get_listener_local_address(
+        &mut self,
+        listener: TcpListener,
+    ) -> HostResult<IpSocketAddress, Errno> {
+        todo!()
+    }
+
+    async fn get_tcp_connection_local_address(
+        &mut self,
+        connection: TcpConnection,
+    ) -> HostResult<IpSocketAddress, Errno> {
+        todo!()
+    }
+
+    async fn get_remote_address(
+        &mut self,
+        connection: TcpConnection,
+    ) -> HostResult<IpSocketAddress, Errno> {
+        todo!()
+    }
+
+    async fn get_flags(&mut self, connection: TcpConnection) -> HostResult<ConnectionFlags, Errno> {
+        todo!()
+    }
+
+    async fn set_flags(
+        &mut self,
+        connection: TcpConnection,
+        flags: ConnectionFlags,
+    ) -> HostResult<(), Errno> {
+        todo!()
+    }
+
+    async fn get_receive_buffer_size(
+        &mut self,
+        connection: TcpConnection,
+    ) -> HostResult<Size, Errno> {
+        todo!()
+    }
+
+    async fn set_receive_buffer_size(
+        &mut self,
+        connection: TcpConnection,
+        value: Size,
+    ) -> HostResult<(), Errno> {
+        todo!()
+    }
+
+    async fn get_send_buffer_size(&mut self, connection: TcpConnection) -> HostResult<Size, Errno> {
+        todo!()
+    }
+
+    async fn set_send_buffer_size(
+        &mut self,
+        connection: TcpConnection,
+        value: Size,
+    ) -> HostResult<(), Errno> {
+        todo!()
+    }
+
+    async fn bytes_readable(&mut self, socket: TcpConnection) -> HostResult<Size, Errno> {
         drop(socket);
         todo!()
     }
 
-    async fn bytes_writable(&mut self, socket: Socket) -> HostResult<BytesResult, wasi_tcp::Error> {
+    async fn bytes_writable(&mut self, socket: TcpConnection) -> HostResult<Size, Errno> {
         drop(socket);
         todo!()
     }
+
+    async fn read_via_stream(
+        &mut self,
+        fd: wasi_tcp::TcpConnection,
+    ) -> HostResult<WasiStream, wasi_tcp::Errno> {
+        todo!()
+    }
+
+    async fn write_via_stream(
+        &mut self,
+        fd: wasi_tcp::TcpConnection,
+    ) -> HostResult<WasiStream, wasi_tcp::Errno> {
+        todo!()
+    }
 }