SV-based interfaces for dieing and warning

SV-based interfaces for dieing and warning

New functions croak_sv(), die_sv(), mess_sv(), and warn_sv(), each act
much like their _sv-less counterparts, but take a single SV argument
instead of sprintf-like format and args.  They will accept RVs, passing
them through as such.  This means there's no more need to clobber ERRSV
in order to throw a structured exception.

pp_warn() and pp_die() are rewritten to use the _sv interfaces.
This fixes part of [perl #74538].  It also means that a structured
warning object will be passed through to $SIG{__WARN__} instead of
being stringified, thus bringing warn in line with die with respect to
structured exception objects.

The new functions and their existing counterparts are all fully
documented.