sets: avoid calling countBits32 for 0 (#10619)
sets: avoid calling countBits32 for 0 (#10619)

this speeds up the system.sets time from ~0.2 to ~0.06
in release mode. This is still slower than intsets and
tables (which both are ~0.01).

This assumes that most sets will be sparse.

fixes #10617
dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim