Add a Path type

[robertodr]

In the discussion over #51 it emerged that filesystem paths should be allowed to have richer semantics than that allowed by simple strings.

[robertodr]

I don't think a specific path type is needed. Casting to a Python Path is actually counterproductive, as it can be natively serialized to JSON. Even that were possible, deserialization in any other language would surely lead to surprises. A case-sensitive string type would capture the need to skip case normalization and checks on paths could still be performed easily.

[bast]

We concluded that we will not do this. We stick with str type for paths and since we do not automatically normalize values, not a problem.