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/cli/modules.c

@@ -43,6 +43,7 @@ extern void stdinIsRawSet(WrenVM* vm);
 extern void stdinIsTerminal(WrenVM* vm);
 extern void stdinReadStart(WrenVM* vm);
 extern void stdinReadStop(WrenVM* vm);
+extern void stdoutFlush(WrenVM* vm);
 extern void schedulerCaptureMethods(WrenVM* vm);
 extern void timerStartTimer(WrenVM* vm);
 
@@ -61,7 +62,7 @@ extern void timerStartTimer(WrenVM* vm);
 // If you add a new class to the largest module below, make sure to bump this.
 // Note that it also includes an extra slot for the sentinel value indicating
 // the end of the list.
-#define MAX_CLASSES_PER_MODULE 5
+#define MAX_CLASSES_PER_MODULE 6
 
 // Describes one foreign method in a class.
 typedef struct
@@ -153,6 +154,9 @@ static ModuleRegistry modules[] =
       STATIC_METHOD("readStart_()", stdinReadStart)
       STATIC_METHOD("readStop_()", stdinReadStop)
     END_CLASS
+    CLASS(Stdout)
+      STATIC_METHOD("flush()", stdoutFlush)
+    END_CLASS
   END_MODULE
   MODULE(os)
     CLASS(Platform)

src/cli/vm.c

@@ -151,7 +151,6 @@ static WrenForeignClassMethods bindForeignClass(
 static void write(WrenVM* vm, const char* text)
 {
   printf("%s", text);
-  fflush(stdout);
 }
 
 static void reportError(WrenVM* vm, WrenErrorType type,

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";