[PATCH] Update SBuf::trim

This replaces the SBuf::trim() to use CharacterSet instead of an SBuf()
list of characters and memchr()

It seems to be faster for CharacterSet lookup than repeated memchr
calls, but Im not certain of that. It is certainly makes simpler parser
code with trim and a predefined CharacterSet than static SBuf set of chars.

Amos