Improve implementation of btrim/ltrim/rtrim: provide a special case for

Improve implementation of btrim/ltrim/rtrim: provide a special case for
single-byte encodings, and a direct C implementation of the single-argument
forms (where spaces are always what gets trimmed).  This is in preparation
for using rtrim1() as the bpchar-to-text cast operator, but is a useful
performance improvement even if we decide not to do that.