Implement support for using the is cached trait with methods

[Kaiepi]

By replacing the routine/method's $!do attribute with its wrapper's
instead of using Routine.wrap (think NativeCall), multi methods are now
possible to make cached, making method support possible to add to the
is cached trait.

Passes make test and make spectest.

[lizmat]

Thank you for this PR. Please note that before Christmas 2015, it was decided to make the is cached trait experimental, because the implementation didn't really do what @larry thought it should do. Since code was depending on that then, it was decided to not remove it. Which is why I haven't done any other work on this.

Even though this would be easy to merge, I would advise against this, at least until we've had a problem-solving discussion on whether we want to continue having the is cached trait in core: if we do, we should probably remove the "experimental` status from it for 6.e.

[Kaiepi]

Besides what @lizmat raised, this doesn't handle the outer context of the wrapped routine properly, so don't merge until that's fixed as well.

[Kaiepi]

Closing this. Will reopen once routine dispatch is sorted out.