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

test/nonlocal/null_before_defined.wren

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

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

test/nonlocal/use_in_function.wren

@@ -1,5 +1,4 @@
 var Global = "global"
-// TODO: Forward reference to global declared after use.
 
 new Fn {
   IO.print(Global) // expect: global

test/nonlocal/use_in_function_before_definition.wren

@@ -0,0 +1,7 @@
+var f = new Fn {
+  IO.print(Global)
+}
+
+var Global = "global"
+
+f.call // expect: global

test/nonlocal/use_in_method.wren

@@ -1,5 +1,4 @@
 var Global = "global"
-// TODO: Forward reference to global declared after use.
 
 class Foo {
   method {

test/nonlocal/use_in_method_before_definition.wren

@@ -0,0 +1,14 @@
+class Foo {
+  method {
+    IO.print(Global)
+  }
+
+  static classMethod {
+    IO.print(Global)
+  }
+}
+
+var Global = "global"
+
+(new Foo).method // expect: global
+Foo.classMethod // expect: global