Implement AsRawHandle on Child

[Félix Saparelli]

Just did it for Tokio: tokio-rs/tokio#3987

While on Unix, having the PID (via Child::id()) is sufficient, on Windows only the handle returned by the process creation has full privileges over the process. As such, it's helpful to implement AsRawHandle for Child.

My immediate usecase is to build up a smol/async-process variant in https://github.com/watchexec/command-group, which supports an async refactor of Cargo Watch. I'm probably going to go with Tokio for that, but for completeness and for other projects which already use async-std I'd love to add an implementation. To do process groups on Windows I use JobObjects, and assign the process handle to the job.

I'm happy to make a PR for it if you approve :)

[Taiki Endo]

It seems to make sense. I would accept PR to add this.