Fix up Meta.eval(_).

- Made it use primitives instead of foreign functions.
- This fixed an issue where the interpreter loop was running re-entrantly.
- Which in turn fixed a GC bug.
- Report a runtime error if the argument isn't a string.
- Report a runtime error if the source doesn't compile.

builtin/meta.wren

@@ -1,3 +1 @@
-class Meta {
-  foreign static eval(source)
-}
+class Meta {}