HPCC-12603 Minor optimizations and introduce bits_t to IBitSet

Also use __builtin_ffs to remove a loop

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
I had to stare at this a bit before realizing that testMask can't be 0, as previously excluded by noMatchMask check..
A comment might be helpful.