Add generic `fmap` operation

[sritchie]

If we do this, and take a tip from clojure.algo.generic, it will be easier to write generic functions that do stuff like replace-tag from sicmutils.differential.

https://github.com/clojure/algo.generic/blob/master/src/main/clojure/clojure/algo/generic/functor.clj

Many types implement something like this, thanks to @littleredcomputer . Getting them together under one roof is a nice move.