Begin finalizing the io::file module

This fleshes out the io::file module a fair bit more, adding all of the functionality that I can think of that we would want. Some questions about the representation which I'm curious about:

* I modified `FileStat` to be a little less platform-agnostic, but it's still fairly platform-specific. I don't want to hide information that we have, but I don't want to depend on this information being available. One possible route is to have an `extra` field which has all this os-dependent stuff which is clearly documented as it should be avoided.

* Does it make sense for directory functions to be top-level functions instead of static methods? It seems silly to import `std::rt::io::file` and `std::rt::io::File` at the top of files that need to deal with directories and files.