Add fast last(), last_mut() and upper_bound() operations…

[dhardy]

…via additional constraints on internal vec

I wrote this as a proof of concept to satisfy a use-case of mine (actually, in my use case elements are never removed so the implementation could be simpler).

This adds three new operations which should all be constant time, at the cost of potentially making remove() and split_off() O(n) operations. Thoughts?

The new operations could of course be added without the other changes, but in that case they would merely be convenience wrappers.

[Gankra]

I don't really care about split_off, but penalizing remove seems pretty harsh.

(If I don't review this further in a few days, please ping me again; my thesis is due very soon!)

[dhardy]

Yes, I think I agree with you (as I said, proof of concept).

Good luck with the thesis.

[Gankra]

WOW this old! I'm gonna close this; feel free to reopen rebase if you're still interested!

[dhardy]

Nope, can't remember why I wanted this.