Move meta and random to "aux" modules.

Wren now has three classes of modules:

- The one magic "core" module that's built in and always needed.
- Auxiliary libraries like "meta" and "random". These do not have any
  dependencies, so can be used even when you embed Wren inside an
  application. But they're also optional and can be disabled if you
  don't need them.
- CLI modules. These ones need libuv and are tied to the CLI wrapper
  around the VM.

builtin/meta.wren

@@ -1,13 +0,0 @@
-class Meta {
-  static eval(source) {
-    if (!(source is String)) Fiber.abort("Source code must be a string.")
-
-    var fn = compile_(source)
-    // TODO: Include compile errors.
-    if (fn == null) Fiber.abort("Could not compile source code.")
-
-    Fiber.new(fn).call()
-  }
-
-  foreign static compile_(source)
-}