Implement AsRawFd for Stdin, Stdout, Stderr, and RawFd

[ids1024]

Is there a reason this is not the case already, or is it just that it is unnecessary for the typical use case and hasn't been added? Would something like this require an RFC, or would a PR suffice?

[sfackler]

This seems fine for Stdin/Stdout/Stderr, but RawFd is just a typedef for c_int, and it seems a bit weird to implement that trait for a number.

[BurntSushi]

Yeah, I'd also like this for Stdin/Stdout/Stderr. I don't think you'd need an RFC for it.

[ids1024]

My use case is for a function with the signature <T: AsRawFd>(stream: &T), which is why I suggest implementing it for RawFd as well. But the Std* types are the relevant ones really.

[codyps]

On RawFd, see rust-lang/rust#41035 . We'd need a RawFd-like that was a wrapper struct to have an AsRawFd impl. This issue notes the need for a newtype: rust-lang/rust#43254 .

[ids1024]

The implementation for Std* is now merged; but not for RawFd, due to the concern mentioned above.