Implement difference method for OrderedSet

This implementation is obvious rather than efficient, but it's efficient
enough for our uses I think. It uses `type(self)` to guarantee that it
works even in subclasses or if the name of the class changes.

mesonbuild/mesonlib.py

@@ -722,3 +722,6 @@ class OrderedSet(collections.MutableSet):
     def update(self, iterable):
         for item in iterable:
             self.__container[item] = None
+
+    def difference(self, set_):
+        return type(self)(e for e in self if e not in set_)