Enable API documentation for `std::os::wasi`.

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.

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