Implicitly define nonlocal names.

If a capitalized name cannot be resolved, a new top-level
variable with its name is implicitly declared. If a real
definition is not found later, a compile time error is raised.

Mutual recursion at the top level works now!

Fix #101. Fix #106.

test/nonlocal/undefined.wren

@@ -0,0 +1,7 @@
+var fn = new Fn {
+  IO.print(Foo)
+  IO.print(Bar)
+}
+
+// expect error line 7
+// expect error line 7