Move Meta to a separate module.

Also cleaned up some of the code around loading the core module.

builtin/meta.wren

@@ -1 +1,13 @@
-class Meta {}
+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)
+}