Expose all OS-specific modules in libstd doc.
1. Uses the special `--cfg dox` configuration passed by rustbuild when running `rustdoc`. Changes the `#[cfg(platform)]` into `#[cfg(any(dox, platform))]` so that platform-specific API are visible to rustdoc.
2. Since platform-specific implementations often won't compile correctly on other platforms, `rustdoc` is changed to apply `everybody_loops` to the functions during documentation and doc-test harness.
3. Since platform-specific code are documented on all platforms now, it could confuse users who found a useful API but is non-portable. Also, their examples will be doc-tested, so must be excluded when not testing on the native platform. An undocumented attribute `#[doc(cfg(...))]` is introduced to serve the above purposed.
Fixes#24658 (Does _not_ fully implement #1998).
Implement fs::rename in sys::redox
This uses a simple implementation of copy + unlink. Redox does not have a rename or link system call for a faster implementation.
Replaced by adding extra imports, adding hidden code (`# ...`), modifying
examples to be runnable (sorry Homura), specifying non-Rust code, and
converting to should_panic, no_run, or compile_fail.
Remaining "```ignore"s received an explanation why they are being ignored.
`Stdio` now implements `From<ChildStdin>`, `From<ChildStdout>`,
`From<ChildStderr>`, and `From<File>`.
The `Command::stdin`/`stdout`/`stderr` methods now take any type that
implements `Into<Stdio>`.
This makes it much easier to write shell-like command chains, piping to
one another and redirecting to and from files. Otherwise one would need
to use the unsafe and OS-specific `from_raw_fd` or `from_raw_handle`.
Leftovers from #39594; From<Box> impls
These are a few more impls that follow the same reasoning as those from #39594.
What's included:
* `From<Box<str>> for String`
* `From<Box<[T]>> for Vec<T>`
* `From<Box<CStr>> for CString`
* `From<Box<OsStr>> for OsString`
* `From<Box<Path>> for PathBuf`
* `Into<Box<str>> for String`
* `Into<Box<[T]>> for Vec<T>`
* `Into<Box<CStr>> for CString`
* `Into<Box<OsStr>> for OsString`
* `Into<Box<Path>> for PathBuf`
* `<Box<CStr>>::into_c_string`
* `<Box<OsStr>>::into_os_string`
* `<Box<Path>>::into_path_buf`
* Tracking issue for latter three methods + three from previous PR.
Currently, the opposite direction isn't doable with `From` (only `Into`) because of the separation between `liballoc` and `libcollections`. I'm holding off on those for a later PR.
