Get static methods fully working.

They are compiled as local variables defined in an implicit
scope surrounding the class. This has the right semantics and,
better, means that there's no VM support needed for them.

They're purely syntax sugar.

test/static_field/use_before_set.wren

@@ -0,0 +1,8 @@
+class Foo {
+  static write { IO.print(__field) } // Compile a use of the field...
+  static init { __field = "value" }  // ...before an assignment to it.
+}
+
+// But invoke them in the right order.
+Foo.init
+Foo.write // expect: value