Add a new function Perl_hv_common_key_len(), which contains the

Add a new function Perl_hv_common_key_len(), which contains the
manipulations to convert negative lengths to positive length + UTF-8
flag. hv_delete(), hv_exists(), hv_fetch(), hv_store() and
hv_store_flags() all become mathoms. The macros hv_fetchs() and
hv_stores() call hv_common() directly.

p4raw-id: //depot/perl@31931