Generalize implicit navs to function like dynamic navs

[nathanmarz]

Right now implicit navs must return a RichNavigator implementation. They would be more flexible if generalized so that:

• An implicit nav can be returned (and then further "expanded")
• Dynamic values can be in the return (so an implicit nav can use local parameters)

An example implicit nav you might want to build with these features is translating a map to multi-path + keypath, e.g.:

(transform {:a (nthpath i) :b STAY} inc {:a [1 2 3] :b 4})
;; => {:a [1 3 3] :b 5}