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",
 ]

README.md

@@ -18,3 +18,6 @@ sent off-platform to the tenant, who can verify the Quote with a certificate cha
 
 ## [Wasmtime - Simple Custom Syscall](wasmtime-custom-syscall)
 Providing a simple _println_ syscall to a pre-compiled WASM binary written in Rust.
+
+## [Wasmtime - UDP Echo-Server](wasmtime-udp-echoserver)
+Providing a rudimentary UDP implementation to a pre-compiled WASM binary written in Rust, which implements a UDP Echo-Server on top of the provided syscalls.