feat: Add zipWithNext

Description

Adds a zipWithNext function, similar to kotlin's.

I cannot add the transform parameter because dart doesn't support overloading, but it can be easily replaced by chaining this with map.

I added a new optional parameter to close the loop that I found useful. That is not kotlin standard though, and I can remove.

Example usage: convert list of vertices to list of edges of a polygon.

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In order to facilitate testing, I added equals & hashcode impl to the Pair class. I think that is overall useful as I expect Pair to be a "data class" and delegate equals to the types of first/second.

However that is not mandatory for this change, if you don't like it I can revert it and add extra code on the tests to properly compare two Pairs.

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