wasmtime: add net syscalls for an udp echo server

This demo shows the extension of the previously introduced _stdio_ syscall
module, and adds a _net_ syscall module, which implements rudimentary
UDP functionality.

The WASM guest application uses these syscalls to build an UDP echo
server.

Author: steveej

Cargo.toml

@@ -6,4 +6,5 @@ members = [
     "wasmtime-basic",
     "wasmtime-native-embed",
     "wasmtime-custom-syscall",
+    "wasmtime-udp-echoserver",
 ]