sys: reveal std::io representation to sys module

This commit adds a `AsInner` trait to `sys_common` and provides
implementations on many `std::io` types. This is a building block for
exposing platform-specific APIs that hook into `std::io` types.
libs: add std::os::unix module

The new `std::os::unix` module exposes several extension traits
for extracting file descriptors from `std::io` types.
libs: add std::os::windows module

The new `std::os::windows` module exposes several extension traits
for extracting file descriptors, sockets, and handles from `std::io`
types.