Remove redundant sort

This speeds up Set creation by about 33%
Merge pull request #248 from hoelzro/nom

Remove redundant sort
Lazily initialize $!WHICH in Set

$!WHICH is only needed if we use a Set as a hash key, but
we pay the penalty for sorting the members on every set construction
Merge pull request #249 from hoelzro/nom

Lazily initialize $!WHICH in Set