Walkable neighbors lists should only be updated locally

[alice-i-cecile]

This is currently updated globally, as correctness is easier to reason about this way.

However, there's no need to do this: only the tile and it's neighborhood should ever change.

This is a tricky change: we probably want more tests before attempting it in earnest.

[alice-i-cecile]

The quadratic scaling impact is particularly severe during world generation.