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/closure.wren

@@ -0,0 +1,16 @@
+class Foo {
+  static initialize { __field = "Foo field" }
+
+  static closeOverGet {
+    return fn { return __field }
+  }
+
+  static closeOverSet {
+    return fn { __field = "new value" }
+  }
+}
+
+Foo.initialize
+IO.print(Foo.closeOverGet.call) // expect: Foo field
+Foo.closeOverSet.call
+IO.print(Foo.closeOverGet.call) // expect: new value