[boost] [string_algo] trim_left should be trim_head

After reading Jeff's post about super_string, it suddenly hit me: on RTL languages, the head of the string is actually the right side, so trim_left/trim_right should really be trim_head/trim_tail... I know that the left/right names for trimming are de-facto standard. In fact, they are so rooted, that even a native Hebrew speaker like me didn't get it until just now... But I think that if we want to be 100% accurate, then the head/tail names are more correct. Yuval