tokio-io: impl asRawFd/AsRawHandle for stdio

[MarinPostma]

fixes: #2311

provides an implementation of AsRawFd and AsRawHandle for Stdin, Stdout and Stderr to match the std counterpart behavior.

Motivation

It is sometimes necessary to manipulate stdio as raw file descriptors, to set attributes for example.

Solution

The Std* is wrapped in an Option in a Blocking, which makes it impossible to take it out in a non blocking way. I implemented it the same way Async-std did it, and retrieve another instance of std::io::Std*, and use it to retrieve an new RawFd.

[carllerche]

Thanks 👍