add hfree_next_entry(), hv_free_ent_ret()

add hfree_next_entry(), hv_free_ent_ret()

Move body of hfreeentries()' central loop into a new function,
hfree_next_entry(); leaving hfreeentries() as a simple loop that calls
hfree_next_entry() until there are no entries left.

This will in future allow sv_clear() to free a hash iteratively rather
than recursively.

Similarly, turn hv_free_ent() into a thin wrapper around a new function,
hv_free_ent_ret(), which doesn't free HeVAL(), but rather just returns the
SV instead.