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)
Just a reminder that `Vec::push_all` has not been deprecated because of ergonomics and performance problems and that, at this point, using `extend` is much, much slower than using `push_all`.
```rust
test extend   ... bench:      2868 ns/iter (+/- 9)
test push_all ... bench:        76 ns/iter (+/- 0)
```
Have any concrete plans been made to fix this?