Support zero-copy data in libdeno.send(). (#838)

This is a large API refactor of deno.h which replaces
deno_send() and deno_set_response() with deno_respond().
It also adds a req_id parameter to the deno_recv_cb.

Make writeFile/writeFileSync use it.

js/libdeno.ts

@@ -6,7 +6,7 @@ type MessageCallback = (msg: Uint8Array) => void;
 interface Libdeno {
   recv(cb: MessageCallback): void;
 
-  send(msg: ArrayBufferView): null | Uint8Array;
+  send(control: ArrayBufferView, data?: ArrayBufferView): null | Uint8Array;
 
   print(x: string, isErr?: boolean): void;