Implement more native file I/O

This implements a fair amount of the unimpl() functionality in io::native
relating to filesystem operations. I've also modified all io::fs tests to run in
both a native and uv environment (so everything is actually tested).

There are a two bits of remaining functionality which I was unable to get
working:

* change_file_times on windows
* lstat on windows

I think that change_file_times may just need a better interface, but lstat has a
large implementation in libuv which I didn't want to tackle trying to copy.