sv.c: Add S_sv_freshcopy_flags and S_sv_freshcopy_POK

S_sv_freshcopy_flags is a drop-in replacement for `Perl_sv_setsv_flags`.
It is designed for use when the destination SV is being freshly created
and much of the logic in `sv_setsv_flags` is irrelevant.

The intended users for this new function are:
* Perl_sv_mortalcopy_flags
* Perl_newSVsv_flags

Those functions have been modified such that:
* Bodyless destination SVs are created inline
* SVt_PVs also have special casing
* SVt_PVMG and below use S_sv_freshcopy_flags
* Anything else drops back to using Perl_sv_setsv_flags

S_sv_freshcopy_POK is a helper function that concentrates on the string
assignment logic:
* Swipe the buffer
* CoW the buffer
* Copy the buffer

Using perl's test harness as a guide:
* 45% of Perl_newSVsv_flags / 57% of Perl_sv_mortalcopy_flags calls use
  the bodyless code
* 32% of Perl_newSVsv_flags / 36% of Perl_sv_mortalcopy_flags calls use
  the SVt_PV/SVp_POK code
* The S_sv_freshcopy_flags code handles the bulk of the remainder.

With these changes compared with a build of blead:

* `perl -e 'for (1..100_000) { my $x = [ (1) x 1000 ]; }'` runs 30% faster

* `perl -e 'for (1..100_000) { my $x = [ ("Perl") x 1000 ]; }' runs:
   * 15% faster if `newSV_type(SVt_PV)` is NOT inlined
   * 30% faster if it IS inlined

The overall reduction in branches when cloning SVs, and refocusing of branch
prediction within Perl_sv_setsv_flags, will hopefully give a meaningful
boost to realistic Perl applications.

Author: richardleach