This PR adds some internal infrastructure to allow the private `std::sys` module to access internal representation details of `std::io`.
It then exposes those details in two new, platform-specific API surfaces: `std::os::unix` and `std::os::windows`.
To start with, these will provide the ability to extract file descriptors, HANDLEs, SOCKETs, and so on from `std::io` types.
More functionality, and more specific platforms (e.g. `std::os::linux`) will be added over time.
Closes#18897