Move `std::sys::unix::platform` to `std::sys::unix::ext`
This moves the operating system dependent alias `platform` (`std::os::{linux, android, ...}`) from `std::sys::unix` to `std::sys::unix::ext` (a.k.a. `std::os::unix`), removing the need for compatibility code in `unix_ext` when documenting on another platform.
This is also a step in making it possible to properly move `std::sys::unix::ext` to `std::os::unix`, as ideally `std::sys` should not depend on the rest of `std`.
This adds API documentation support for `std::os::wasi` modeled after
how `std::os::unix` works, so that WASI can be documented [here] along
with the other platforms.
[here]: https://doc.rust-lang.org/stable/std/os/index.html
Two changes of particular interest:
- This changes the `AsRawFd` for `io::Stdin` for WASI to return
`libc::STDIN_FILENO` instead of `sys::stdio::Stdin.as_raw_fd()` (and
similar for `Stdout` and `Stderr`), which matches how the `unix`
version works. `STDIN_FILENO` etc. may not always be explicitly
reserved at the WASI level, but as long as we have Rust's `std` and
`libc`, I think it's reasonable to guarantee that we'll always use
`libc::STDIN_FILENO` for stdin.
- This duplicates the `osstr2str` utility function, rather than
trying to share it across all the configurations that need it.
