The windows/unix modules were currently inconsistent about the traits being
implemented for `DirEntry` and there isn't much particular reason why the traits
*couldn't* be implemented for `ReadDir` and `DirEntry`, so this commit ensures
that they are implemented.
Closes#22577
Per [RFC 579](https://github.com/rust-lang/rfcs/pull/579), this commit
adds a new `std::process` module. This module is largely based on the
existing `std::old_io::process` module, but refactors the API to use
`OsStr` and other new standards set out by IO reform.
The existing module is not yet deprecated, to allow for the new API to
get a bit of testing before a mass migration to it.
This commit is an implementation of [RFC 739][rfc] which adds a new `std::fs`
module to the standard library. This module provides much of the same
functionality as `std::old_io::fs` but it has many tweaked APIs as well as uses
the new `std::path` module.
[rfc]: https://github.com/rust-lang/rfcs/pull/739
