pp.c:pp_rv2sv: Use >= instead of switch

This results in smaller machine code.

If a freed SV is exposed to perl space then ${\$freed} will now die,
which is arguably more correct.
pp.c:pp_pos: Less stack fiddling

We always pop exactly one item off the stack and then push exactly one
item.  Instead of doing sp-- and sp++, just leave it alone (TOP/SET,
rather than POP/PUSH).  Also skip PUTBACK, which is unnecessary.
pp.c:pp_rv2cv: Remove PUTBACK

pp_rv2cv doesn’t adjust the stack pointer at all, so there is no need
to synchronise with the global pointer before returning.