Unify lex_cast* and avoid string copies

Unify lex_cast* and avoid string copies

We always use lex_cast either to string or from string, so I made that
explicit, and got rid of some string copies in the process.  There was
also stringify(), which was redundant to lex_cast<string>.

We also always used lex_cast_hex to string, so that's now hard-coded and
again eliminated a string copy.

For lex_cast_qstring<string>, there's no need to write the streamify the
input, so a specialization now operates directly on the input.

Hopefully this is a bit cleaner, and I do measure it to be a little
faster on scripts with many probes.