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/null_before_defined.wren

@@ -0,0 +1,3 @@
+IO.print(Foo) // expect: null
+var Foo = "value"
+IO.print(Foo) // expect: value