Use "Builder"s to implement some Set and Map construction functions.
As a result, some have become good consumers in terms of list fusion,
and all are now O(n) for non-decreasing input.

                     Fusible  Fusible  O(n) for     O(n) for
                     before   after    before       after
Set.fromList         No       Yes      Strict incr  Non-decr
Set.map              -        -        Strict incr  Non-decr
Map.fromList         No       Yes      Strict incr  Non-decr
Map.fromListWith     Yes      Yes      Never        Non-decr
Map.fromListWithKey  Yes      Yes      Never        Non-decr
Map.mapKeys          -        -        Strict incr  Non-decr
Map.mapKeysWith      -        -        Never        Non-decr