unify PERL_HASH and PERL_HASH_INTERNAL

unify PERL_HASH and PERL_HASH_INTERNAL

Those two macros expand into two large, almost identical chunks of code.
The only difference between the two is the source of the hash seed.
So parameterize this into a new PERL_HASH_INTERNAL_() macro.

Also, there are a couple of places in hv.c that do the rough equivalent of

    if (HvREHASH(hv))
	key = PERL_HASH_INTERNAL(...)
    else
	key = PERL_HASH(...)

which incorporates two complete macro expansions into the code.
Reorganise them to be

    key = PERL_HASH_INTERNAL_(..., HvREHASH(hv))