require walks to always take a KeyPath object as first argument

[CarloLucibello]

This issue keeps track of the breaking change proposed in #74 (review) in order to avoid duplicating walks, e.g. defining SomeWalk and also SomeWalkWithPath.

[ToucheSir]

I realize this is the second time I've brought up keyword args today, but could we introduce this change without breaking anything by making the keypath a kwarg of walk constructors?