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>
HPCC-12603 Clean up formatting, and clarify comment

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6659 from ghalliday/issue12603

HPCC-12603 Minor optimizations and introduce bits_t to IBitSet

Reviewed-By: Jake Smith <jake.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>