Have map types return MapEntry instead of vector

Groundwork for this has been done.

CLJS-2013: Add MapEntry typeClosedPreview adds the MapEntry type, but indicates "PAMs and PHMs don't emit them when you seq or iterate over them yet."
CLJS-2068: RedNode, BlackNode and MapEntry should be IComparableClosedPreview fixes an issue with MapEntry
CLJS-2001: Add map-entry? predicateClosedPreview adds a map-entry? predicate

So this ticket entails updating the PHM and PAM implementations so that, for example, (map-entry? (first {:a 1})) evaluates to true.

Note: One additional bit of groundwork that should be done prior to this ticket is CLJS-2460.