auto merge of #10495 : alexcrichton/rust/more-native-io, r=brson

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 few bits of remaining functionality which I was unable to get
working:

* truncate on windows
* change_file_times on windows
* lstat on windows

I think that change_file_times may just need a better interface, but the other
two have large implementations in libuv which I didn't want to tackle trying to
copy. I found a `chsize` function to work for truncate on windows, but it
doesn't quite seem to be working out.

Author: bors

src/libstd/io/buffered.rs

@@ -391,7 +391,6 @@ mod test {
     // newtype struct autoderef weirdness
     #[test]
     fn test_buffered_stream() {
-        use rt;
         struct S;
 
         impl io::Writer for S {