experimental API for safe monkey-patching

 The term "monkey patching" describes injecting additional methods into
 a class whose implementation you don't control. If done without care,
 this is dangerous; the problematic case arises when:
  * You add a method to a class;
  * A newer version of the monkey-patched class adds another method of
    the same name
  * And uses that new method in some other part of its own
    implementation.
 .
 ex::monkeypatched lets you do this sort of monkey-patching safely:
 before it injects a method into the target class, it checks whether the
 class already has a method of the same name. If it finds such a
 method, it throws an exception (at compile-time with respect to the
 code that does the injection).
 .
 See <http://aaroncrane.co.uk/talks/monkey_patching_subclassing/> for
 more details.