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)
rust-wtf8 currently [duplicates some standard library code](https://github.com/SimonSapin/rust-wtf8/blob/master/src/not_quite_std.rs), changing the `char`, `str`, and `String` types to their respective supersets `CodePoint`, `Wtf8`, and `Wtf8Buf`. To avoid the duplication, libstd could maybe have private functions that are generic over these types, so that optimization on monomorphized functions could still take advantage e.g. of the LLVM range asserts on `char`.