Move PL_{No,Yes,hexdigit} from perlvars.h to perl.h, as all are const char[]

Move PL_{No,Yes,hexdigit} from perlvars.h to perl.h, as all are const char[]

They were converted in perl.h from const char[] to #define in 31fb120917c4f65d,
then re-instated as const char[], but in perlvars.h, in 3fe35a814d0a98f4.
There's no need for compile-time constants to jump through the hoops of
perlvars.h, even for Symbian, as the various "EXTCONST" variables already in
perl.h demonstrate.

These were the only 3 users of the the PERLVARISC macro, so eliminate that, and
all related code.