Use of external verified VecMap crate

[jhaye]

This does compile and should work out of the box.

But I want to make some performance testing first, before merging, since I replace every expression that uses closures.

[lhstrh]

@petervdonovan here is another use case for continuous benchmarking.

[jhaye]

Right, so there is basically no performance difference:

There are some detectable differences, but they even out overall. And they are seemingly just a result of already existing benchmark performance variation.