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.

test/meta/eval_compile_error.wren

@@ -0,0 +1 @@
+Meta.eval("!!") // expect runtime error: Could not compile source code.

test/meta/eval_not_string.wren

@@ -0,0 +1 @@
+Meta.eval(123) // expect runtime error: Source code must be a string.