keys(sorteddict) is not an AbstractSet

As of #787, the iterators for the sorted containers have been unified into a single framework that handles all three containers, full containers or subranges, keys/values/both, tokens/semitokens, and forward/reverse. There is a remaining issue. If sd is a SortedDict{K,V}, then the type of keys(sd) lives in the new framework and is not a subtype of AbstractSet{K}. This behavior is contrary to keys(d) where d is a Dict{K,V}; the latter is a subtype of AbstractSet{K}. I don't know how to fix this, aside from disuniting the new iterator framework to implement keys(sd) separately. I asked about this on Discourse (https://discourse.julialang.org/t/possible-to-make-a-specific-parameterization-into-a-subtype/84260) but did not receive any responses.

Start with the unified iterator framework for sorted containers and the keys(sd) entry point, comparing its type with keys(d) for Dict. Done means keys(sd) is a subtype of AbstractSet{K} while retaining the sorted-container iterator behavior described in the issue.