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.

*Thanks to @kballard, who helped spark some of the initial ideas over Ike's sandwiches!*

[Rendered](https://github.com/aturon/rfcs/blob/path-reform/text/0000-path-reform.md)
I don’t mind “UCS-2” semi-informally in text, but I’d rather not have `ucs2` in API names. Also, Microsoft uses “UTF-16” in its documentation, never “UCS-2”. The most accurate is “potentially ill-formed UTF-16”, but that’s too long for a method name. So I’d rather have the method names be `from_utf16` and `to_utf16` with the documentation noting that it can be ill-formed / invalid UTF-16.