Improve the Extend::extend implementation for performances

[Kerollmops]

Fixes #301.

[irevoire]

The code seems right to me. It could execute way quickly if the iterator was sorted, but I guess that can wait until we need it 😅

My only concerns is that, from what I understand, this function is exactly matching the API of the Extend trait.
Shouldn't we implement that trait instead of making our own function?

[Kerollmops]

You are absolutely right that the insert_many method should be replaced by the Extend::extend one. I changed the implementation of the extend method to be the same as the insert_many one and removed the latter.

Thank you 🙏

[irevoire]

One last smol change and we can merge!

[irevoire]

Perfecto 👌