API changes for FSet 2

[slburson]

Proposed:

• lookup on a bag should return the multiplicity
• [under consideration] update should be renamed (maybe to update-path?) so the name can be used for a GF that updates a map (or seq, or tuple) value without duplicating the walk to the value; see WB-Map-Tree-Update. In any case, the existing update is probably very rarely used.
• more to follow...