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)
@huonw Given https://github.com/rust-lang/rfcs/pull/447, this will probably have to move inward, but at the moment this seemed the most concise way to present the API.