Don't automatically flush on every System.write().

It's handy because it ensures writes are flushed to the terminal before
any calls to read from stdin, but it's also gratuitously slow.

Instead, added a Stdout class with an explicit flush() method that can
be called by the user.

Fix #445.

src/module/io.c

@@ -545,6 +545,12 @@ void stdinIsTerminal(WrenVM* vm)
   wrenSetSlotBool(vm, 0, uv_guess_handle(stdinDescriptor) == UV_TTY);
 }
 
+void stdoutFlush(WrenVM* vm)
+{
+  fflush(stdout);
+  wrenSetSlotNull(vm, 0);
+}
+
 static void allocCallback(uv_handle_t* handle, size_t suggestedSize,
                           uv_buf_t* buf)
 {

src/module/io.wren

@@ -297,3 +297,7 @@ class Stdin {
   foreign static readStart_()
   foreign static readStop_()
 }
+
+class Stdout {
+  foreign static flush()
+}

src/module/io.wren.inc

@@ -298,4 +298,8 @@ static const char* ioModuleSource =
 "\n"
 "  foreign static readStart_()\n"
 "  foreign static readStop_()\n"
+"}\n"
+"\n"
+"class Stdout {\n"
+"  foreign static flush()\n"
 "}\n";