native: Improve windows file handling

This commit splits the file implementation into file_unix and file_win32. The
two implementations have diverged to the point that they share almost 0 code at
this point, so it's easier to maintain as separate files.

The other major change accompanied with this commit is that file::open is no
longer based on libc's open function on windows, but rather windows's CreateFile
function. This fixes dealing with binary files on windows (test added in
previous commit).

This also changes the read/write functions to use ReadFile and WriteFile instead
of libc's read/write.

Closes #12406

src/libnative/io/pipe_unix.rs

@@ -17,8 +17,8 @@ use std::rt::rtio;
 use std::sync::arc::UnsafeArc;
 use std::intrinsics;
 
-use super::{IoResult, retry};
-use super::file::{keep_going, fd_t};
+use super::{IoResult, retry, keep_going};
+use super::file::fd_t;
 
 fn unix_socket(ty: libc::c_int) -> IoResult<fd_t> {
     match unsafe { libc::socket(libc::AF_UNIX, ty, 0) } {