Adds support for type-fn multimethods

[jelmerderonde]

This PR makes it possible to have multimethods as type-fns. It's pretty basic, I didn't think any more changes were needed. Let me know If there's something else to do.

[vlaaad]

I've been thinking about it, and what worries me is that it might be a breaking change. Users might have their own fx->lifecycle function like that:

{:fx.opt/type->lifecycle #(or (fx/keyword->lifecycle %)
                              (fx/fn->lifecycle %)
                              (when (instance? clojure.lang.MultiFn %)
                                my-custom-multi-fn-lifecycle))}

That way after update their apps will break because MultiFns will suddenly start using different lifecycles.
I think a non-breaking way to add MultiFn support is somehow checking for MultiFns after using type->lifecycle. It could be done by extending the Lifecycle protocol to multi-methods.

I just realized there is another problem: applications that use contexts have different lifecycles for functions, so there is another place to update: fx/fn->lifecycle-with-context. Sorry, I didn't realize that adding support for multi-methods is much more complex...

[vlaaad]

I'll need some more time to think about MultiFn support, extending Lifecycle protocol won't work for them since there needs to be 2 different behaviors depending on whether contexts are used or not.