make hv freeing iterative rather than recursive

make hv freeing iterative rather than recursive

make sv_clear() iteratively free the elements of a hash, rather than
recursing. This stops freeing deeply nested hash of hash structures from
blowing the stack.

This commit is relatively straightfoward, now that
a) the infrastruure is already in place in sv_clear to iteratively
   free AVs;
b) the long sequence of commits leading up to this has provided us with
    the hfree_next_entry() function, which returns just the next sv in the
    hash that needs freeing.

When starting to free a new hash, we have to save the old value of iter_sv
somewhere; we do this by sticking it in the unused SvSTASH slot of the HV.
This slot shouldn't get messed with, since, but this time we've already
called the destructor on this object, and we should have a refcount of
zero, so no destructor should be able to see us to rebless us.

Ideally we'd like to be able to save the old index into HvARRAY when
freeing a new HV, but I couldn't think of anywhere to hide it.
So we get sub-optimal scanning of the parent's HvARRAY when freeing hashes
of hashes.