Summary:
Fix hex2String performance issues by removing sscanf dependency.
Also fixed some edge case handling (odd length, bad input).

Test Plan: Created a test file which called old and new implementation, and validated results are the same. I'll paste results in the phabricator diff.

Reviewers: igor, rven, anthony, IslamAbdelRahman, kradhakrishnan, yhchiang, sdong

Reviewed By: sdong

Subscribers: thatsafunnyname, leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D46785