add hexlify() and unhexlify() convenience methods

add hexlify() and unhexlify() convenience methods

Summary:
Add new versions of hexlify() and unhexlify() that directly return a
std::string, similar to the helper functions that already exist for humanify()
and backslashify().

The function signatures for these helpers are different than the humanify() and
backslashify() helpers--these functions always accept StringPiece or ByteRange
arguments.  This allows them to easily accept "const char*" arguments, without
having to convert the arguments to a std::string.  (I think we probably should
fix the humanify() and backslashify() helpers to be similar in a separate
diff.)

Reviewed By: yfeldblum

Differential Revision: D3403863

fbshipit-source-id: a2df49d5857e1d34caac3d78283dc293f4ef1ab6