Fix dynamic mixins with nested rules

Dynamic mixins with more than one level of nesting wouldn't work.
This is now fixed. Also refactored mixin.definition.eval a little.

test/css/mixins-nested.css

@@ -0,0 +1,6 @@
+.class .inner {
+  height: 300;
+}
+.class .inner .innest {
+  width: 30;
+}