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.

src/libstd/io/stdio.rs

@@ -316,12 +316,22 @@ impl Writer for StdWriter {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use rt::test::run_in_newsched_task;
 
     #[test]
-    fn smoke() {
+    fn smoke_uv() {
         // Just make sure we can acquire handles
         stdin();
         stdout();
         stderr();
     }
+
+    #[test]
+    fn smoke_native() {
+        do run_in_newsched_task {
+            stdin();
+            stdout();
+            stderr();
+        }
+    }
 }