RFC: path reform

This RFC reforms the design of the `std::path` module in preparation for API
stabilization. The path API must deal with many competing demands, and the
current design handles many of them, but suffers from some significant problems
given in "Motivation" below. The RFC proposes a redesign modeled loosely on the
current API that addresses these problems while maintaining the advantages of
the current design.

[Rendered](https://github.com/aturon/rfcs/blob/path-reform/text/0000-path-reform.md)
Right, I should've talked about this. Right now we generally panic in that case, and I think we should continue with that approach. But it's not entirely clear whether to do the inspection at the path level or only when crossing into fs. (In any case, I expect the signature to stand.)