use std::string to implement make_shorty
use std::string to implement make_shorty

Summary: Replace std::ostringstream with std::string to save cpu cycles.

Reviewed By: NTillmann

Differential Revision: D16597341

fbshipit-source-id: 62f6a338db41ec5abd740f03496543819057dd3b

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex