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/mutual_recursion.wren

@@ -0,0 +1,10 @@
+class Foo {
+  static bar { new Bar }
+}
+
+class Bar {
+  static foo { new Foo }
+}
+
+IO.print(Foo.bar) // expect: instance of Bar
+IO.print(Bar.foo) // expect: instance of Foo