Add map function

[BridgeTheMasterBuilder]

Hi. Thanks for making this package, I have been relying heavily on it for a project I've been working on. The lack of a map function and having to simulate it however represented a bottleneck so I added an ability to map over the bindings, which seems conspicuously missing anyway as both fold and filter are there.

I am not completely familiar with the data structure so I just modeled it after filter. In any case, it behaves as expected.