Native thread-blocking I/O implementation

This module contains the implementation of native thread-blocking implementations of I/O on all platforms. This module is not intended to be used directly, but rather the rust runtime will fall back to using it if necessary.

Rust code normally runs inside of green tasks with a local scheduler using asynchronous I/O to cooperate among tasks. This model is not always available, however, and that's where these native implementations come into play. The only dependencies of these modules are the normal system libraries that you would find on the respective platform.