A couple minor tweaks to the CloneMerge interface

While I was updating to the latest release and clearing clips lints, I noticed a couple minor changes that may be worth making. I can PR this later but I wanted to make an issue so I don't forget:
- [ ] Mark CloneImpl::clone_components as unsafe. Rationale being, the implementor will almost certainly be dereferencing raw pointers. So they are likely to get clips lints they have to suppress (#[allow(clippy::not_unsafe_ptr_arg_deref)])
- [ ] Make SpawnCloneImpl generic over the hash map's hash function. I generally use the fnv crate for all hashmaps in my codebase but cannot do that with the current API.

I'd like to implement this locally and verify that the changes don't cause any unexpected issues in my code downstream of this feature.

(p.s. - congrats on the recent new release!)