Replace bitwise looping with bytewise looping in hemdistsign and

Replace bitwise looping with bytewise looping in hemdistsign and
sizebitvec of tsearch2, as well as identical code in several other
contrib modules.  This provided about a 20X speedup in building a
large tsearch2 index ... didn't try to measure its effects for other
operations.  Thanks to Stephan Vollmer for providing a test case.