Feature request: Heterogeneous overloads for insertion.

[BenKaufmann]

tsl::hopscotch_map already provides heterogeneous overloads for lookup (e.g. via find or at) but there is currently no such support for adding elements (e.g. via operator[] or try_emplace). AFAIK, corresponding overloads were added to
the standard associative containers with C++26.

@Tessil Do you think it makes sense to also extend tsl::hopscotch_map? From what I can see, it seems that the underlying tsl::hopscotch_hash already has the necessary functionality, so I drafted a first version for try_emplace and operator[] here. I only extended tsl::hopscotch_map as tsl::bhopscotch_map seems to sometimes copy keys anyways.

If you think this makes sense, I could also open a PR.

[BenKaufmann]

I created #63 just in case you think it could be useful.

[Tessil]

Thank you for the contribution, I'll try to take a look this weekend.

I didn't know it'd be part of C++26, I'll check a bit the proposal.