Implement more native file I/O

[alexcrichton]

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.

[andrew-d]

@alexcrichton If you have a native HANDLE on Windows, you can use [SetFilePointerEx](http://msdn.microsoft.com/en-us/library/windows/desktop/aa365542(v=vs.85\).aspx) and [SetEndOfFile](http://msdn.microsoft.com/en-us/library/windows/desktop/aa365531(v=vs.85\).aspx) to truncate, FWIW.

[andrew-d]

Just building on this: you can use [_get_osfhandle](http://msdn.microsoft.com/en-us/library/aa297958(v=vs.60\).aspx) to get the underlying HANDLE, and then use the above Win32 APIs.

[alexcrichton]

@andrew-d, amazing! I will implement it as such before merging this.

[alexcrichton]

Thanks to @andrew-d's suggestion, truncate is now implemented!

[thestinger]

By the way, we could outsource a lot of these Windows porting efforts by switching to mingw-w64. They have a much larger suite of POSIX functions implemented than mingw (including ones like ftruncate), and unlike mingw they actually work properly :P.

[alexcrichton]

If we dropped the mingw dependency completely, would we still have the old POSIX functionality implemented natively by windows?

[thestinger]

No, we wouldn't, but we can't drop the mingw runtime if we don't want to make a C99 math library or find an alternative (and more). Unlike the legacy mingw, mingw-w64 is a solid and very actively developed runtime providing the best C++11 support on Windows via gcc.

[alexcrichton]

utime moved into rust