Add a macro MUTABLE_PTR(p), which on (non-pedantic) gcc will not cast

Add a macro MUTABLE_PTR(p), which on (non-pedantic) gcc will not cast
away const, returning a void *. Add MUTABLE_SV(sv) which uses this, and
replace all (SV *) casts either with MUTABLE_SV(sv), or (const SV *).

This probably still needs some work - assigning to SvPVX() and SvRV()
is now likely to generate a casting error. The core doesn't do this.
But as-is it's finding bugs that can be fixed.

p4raw-id: //depot/perl@34605