"IO" -> "System".

Get rid of the separate opt-in IO class and replace it with a core
System class.

- Remove wren_io.c, wren_io.h, and io.wren.
- Remove the flags that disable it.
- Remove the overloads for print() with different arity. (It was an
  experiment, but I don't think it's that useful.)
- Remove IO.read(). That will reappear using libuv in the CLI at some
  point.
- Remove IO.time. Doesn't seem to have been used.
- Update all of the tests, docs, etc.

I'm sorry for all the breakage this causes, but I think "System" is a
better name for this class (it makes it natural to add things like
"System.gc()") and frees up "IO" for referring to the CLI's IO module.

src/vm/wren_core.wren.inc

@@ -214,4 +214,39 @@ static const char* coreModuleSource =
 "  iteratorValue(iterator) { _map.valueIteratorValue_(iterator) }\n"
 "}\n"
 "\n"
-"class Range is Sequence {}\n";
+"class Range is Sequence {}\n"
+"\n"
+"class System {\n"
+"  static print() {\n"
+"    writeString_(\"\n\")\n"
+"  }\n"
+"\n"
+"  static print(obj) {\n"
+"    writeObject_(obj)\n"
+"    writeString_(\"\n\")\n"
+"    return obj\n"
+"  }\n"
+"\n"
+"  static printAll(sequence) {\n"
+"    for (object in sequence) writeObject_(object)\n"
+"    writeString_(\"\n\")\n"
+"  }\n"
+"\n"
+"  static write(obj) {\n"
+"    writeObject_(obj)\n"
+"    return obj\n"
+"  }\n"
+"\n"
+"  static writeAll(sequence) {\n"
+"    for (object in sequence) writeObject_(object)\n"
+"  }\n"
+"\n"
+"  static writeObject_(obj) {\n"
+"    var string = obj.toString\n"
+"    if (string is String) {\n"
+"      writeString_(string)\n"
+"    } else {\n"
+"      writeString_(\"[invalid toString]\")\n"
+"    }\n"
+"  }\n"
+"}\n";