feat: add Std::Net — TCP sockets module

- C runtime: socket(), bind(), listen(), accept(), connect(), send(), recv(), close()
- library/std/Net.bux: Net_Create, Net_Bind, Net_Listen, Net_Accept, Net_Connect,
  Net_Send, Net_Recv, Net_Close, Net_SetReuse, Net_LastError
- examples/tcp_server.bux: echo server on 127.0.0.1:9000
- examples/tcp_client.bux: client that sends 'Hello from Bux!' and prints reply

_test_net/bux.toml

@@ -0,0 +1 @@
+[Package]

_test_net/src/Main.bux

@@ -0,0 +1,5 @@
+import Std::Net;
+func Main() -> int {
+    let fd: int = Net::Create();
+    return 0;
+}