Sorted map navigation

[nathanmarz]

Should be able to navigate to the substructure of a sorted map, like to the first key/value pair, the first value, or to a submap within a range. Could potentially extend FIRST, LAST, and srange for this.

[nathanmarz]

Could use a similar approach as subseq for one of these navigators, e.g. (all-range > :c < :m), (all-range >= :m)