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)
Only semi-related ranting: The current `Show` mechanism seems to kinda have the expectation that only UTF-8 should be emitted, but `Formatter::write` still takes any `&[u8]` bytes. Then `format!` has to do [an UTF-8 check](https://github.com/rust-lang/rust/blob/3e9414463e88f01eef9953f4644ca3ffc3f17250/src/libstd/fmt.rs#L450) (which can `panic!`!) that shouldn’t be necessary.

I think `Show` should be based on Unicode all the way, with `Formatter::write` only accepting `&str`. Maybe I’ll write an RFC.