core: Path typing isn't consistent

[eckter]

Path is an interface that can be used to type Length, Offset, and such.

But we have significant issues with what it actually means. When we handle several types of paths, it generally means "0 = start of the first block included in the path". But there are cases where 0 means the departure point (as in, the travelled path).

That's a significant issue. We should unify this, make the semantics consistent, and document it. We should probably remove the ambiguous Path at least temporarily.