Add util fns like dispatch-1 dispatch-2 etc.

[camsaul]

Functions that can dispatch on the first n values (optionally passing them to another fn) would be nice. e.g. something like

(dispatch-1 keyword) that would be equivalent to (fn [k & _] (keyword k), but with better per-arity optimizations. I have found myself writing these sorts of functions over and over again, and having a set of util fns for common use cases seems like a win.

[camsaul]

In Toucan 2 I named these dispatch-on-first-arg, dispatch-on-first-two-args, etc... maybe those names are a little clearer. TBD