Debug declarations & fontification.

Hi. I have some serious interest in using this, but without the ability to debug the definitions it is pretty much useless. So I've fixed that with some `declare` magic. There's a caveat, namely you have to eval/instrument the definitions in order, but hey, that's a minor inconvenience :P It will also always jump to the last instrumented dispatch handler, so to debug two branches, you need to do it separately.

You can debug both the `defmulti` and `defmethod` (after instrumenting `defmulti` all handlers need to be re-evaled too!)

I've also added the docstrings for `defmulti` (that is a "global" docstring for the method, so no documentation should be added to the handlers)

And the last thing is the fontification for `defmulti`, `defmethod` is for some reason already present in the core.

Cheers, and thanks for the nice package.