In pp_subst, use a mortal scalar for dstr, instead of SAVEFREESV().

In pp_subst, use a mortal scalar for dstr, instead of SAVEFREESV().

Creation of the mortal can be done with newSVpvn_flags(), saving 1 function
call. Also, the mortal stack uses 1 pointer, whereas the save stack will use 2.
It doesn't matter that dstr is now marked SVs_TEMP, as both code paths already
gut it, stealing its SvPVX(). The SV head will happen to last a bit longer now,
to the next FREETMPS, instead of the the scope pop at the end of pp_subst or
pp_substcont.