Make allocator not Clone

[JustForFun88]

@Amanieu Implements the idea proposed in #465 (comment). The A: Allocator + Clone requirement remains only for Clone implementations.

The pull request is basically complete, just a final review (and maybe some tweaks if needed). Also, I haven't documented the functions yet. This can be done after the initial approval of this pull request.

Main changes made:

1. Allacator moved from RawTableInner to RawTable;
2. IS_ZERO_SIZED_TYPE and DATA_NEEDS_DROP constants moved from separate structures to SizedTypeProperties trait;
3. For ease of implementation and avoidance of duplication, some functions are downgraded from RawTable to RawTableInner. These are: with_capacity, iter, drop_elements. Additionally, the drop_inner_table function has been created through which impl Drop for RawTable<T, A> is now implemented.

[JustForFun88]

If you're happy with the implementation, I can start documenting the changes I've made.

Upd 1: I'm working on documenting functions. I'll probably finish tomorrow.

@Amanieu I have made all you requests and document my all changes. I also made additional changes compared to the previous review: using const RawTableInner::NEW instead of RawTableInner::new();

In addition, I made the reserve_rehash function unsafe, since it was incorrectly labeled as safe from the start.

Could you please review the PR again?

[Amanieu]

@bors r+

[bors]

📌 Commit 91d3675 has been approved by Amanieu

It is now in the queue for this repository.

[bors]

⌛ Testing commit 91d3675 with merge fe81cad...

[bors]

☀️ Test successful - checks-actions
Approved by: Amanieu
Pushing fe81cad to master...