This commit starts to organize the `std::os::$platform` modules and in the
process stabilizes some of the functionality contained within. The organization
of these modules will reflect the organization of the standard library itself
with extension traits for primitives in the same corresponding module.
The OS-specific modules will grow more functionality over time including
concrete types that are not extending functionality of other structures, and
these will either go into the closest module in `std::os::$platform` or they
will grow a new module in the hierarchy.
The following items are now stable:
* `os::{unix, windows}`
* `unix::ffi`
* `unix::ffi::OsStrExt`
* `unix::ffi::OsStrExt::{from_bytes, as_bytes, to_cstring}`
* `unix::ffi::OsString`
* `unix::ffi::OsStringExt::{from_vec, into_vec}`
* `unix::process`
* `unix::process::CommandExt`
* `unix::process::CommandExt::{uid, gid}`
* `unix::process::ExitStatusExt`
* `unix::process::ExitStatusExt::signal`
* `unix::prelude`
* `windows::ffi`
* `windows::ffi::OsStringExt`
* `windows::ffi::OsStringExt::from_wide`
* `windows::ffi::OsStrExt`
* `windows::ffi::OsStrExt::encode_wide`
* `windows::prelude`
The following items remain unstable:
* `unix::io`
* `unix::io::{Fd, AsRawFd}`
* `unix::fs::{PermissionsExt, OpenOptionsExt}`
* `windows::io`
* `windows::io::{Handle, AsRawHandle}`
* `windows::io::{Socket, AsRawSocket}`
* `windows::fs`
* `windows::fs::OpenOptionsExt`
Due to the reorgnization of the platform extension modules, this commit is a
breaking change. Most imports can be fixed by adding the relevant libstd module
in the `use` path (such as `ffi` or `fs`).
[breaking-change]
This commit moves `std::env` away from the `std::old_io` error type as well as
the `std::old_path` module. Methods returning an error now return `io::Error`
and methods consuming or returning paths use `std::path` instead of
`std::old_path`. This commit does not yet mark these APIs as `#[stable]`.
This commit also migrates `std::old_io::TempDir` to `std::fs::TempDir` with
essentially the exact same API. This type was added to interoperate with the new
path API and has its own `tempdir` feature.
Finally, this commit reverts the deprecation of `std::os` APIs returning the old
path API types. This deprecation can come back once the entire `std::old_path`
module is deprecated.
[breaking-change]
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.
