Add switcher

[paf31]

TODO:

• [x] Make key events "hot"
• [x] Move switcher and fixE into IsEvent (or some other classes)
• [x] Documentation and semantics

Notes:

• Events support cancellation now
• It is preferable to have behaviors be "hot" now in the sense that their underlying events are always on. Otherwise switcher might cause them to get reconnected, which can cause blips (consider the move event which returns Nothing until we see the first mousemove event).

@i-am-tom @gabejohnson Any thoughts on this API change?

[paf31]

Ok, this is ready for a final review before merging, by anyone who is interested and motivated to do a review 😄

[gabejohnson]

👍 👍 on swapping mapMaybe out for a Filterable constraint.