Impl AsRawFd for tokio::fs::File

[film42]

Hey! This is my first open source rust PR to a major project, so please let me know if anything looks wrong, or if there's a better practice to implement this!

There are cases where a user may want to grab the underlying fd of a
tokio::fs::File, example: if a user wanted to use a non-blocking splice
copy to the file, they would need to be able to pull out the RawFd.
Since std::fs::File implements this, it seems like a safe thing to
expose from tokio::fs::File.

Motivation

I was implementing a non-blocking splice copy, but found that tokio::fs::File does not implement AsRawFd which was surprising since std::fs::File implements this. I thought it would be nice to add here as well.

Solution

The solution is pretty easy since the underlying std File type already implements this.

[film42]

Whoops, just saw #1640 already implements this and is waiting for a review. I'll close this one.