Reorder NamedPipe fields

Moving the Overlapped fields to the start to make it easier to determine
the offsets and hopefully incur less breakage once external fields
change size.

Note that the Overlapped fields internally uses miow::Overlapped, which
in turn is a OVERLAPPED struct as found in the winapi crate and has a
stable layout (as defined by the Windows API).

src/sys/windows/named_pipe.rs

@@ -70,23 +70,19 @@ pub struct NamedPipe {
 /// constants depend on it, see the `offset_constants` test.
 #[repr(C)]
 struct Inner {
-    handle: pipe::NamedPipe,
-
     connect: Overlapped,
-    connecting: AtomicBool,
-
     read: Overlapped,
     write: Overlapped,
-
+    handle: pipe::NamedPipe,
+    connecting: AtomicBool,
     io: Mutex<Io>,
-
     pool: Mutex<BufferPool>,
 }
 
 /// Offsets from a pointer to `Inner` to the `connect`, `read` and `write`
 /// fields.
-const CONNECT_OFFSET: usize = size_of::<pipe::NamedPipe>(); // `handle` field.
-const READ_OFFSET: usize = CONNECT_OFFSET + size_of::<Overlapped>() + size_of::<usize>(); // `connect`, `connecting` (`bool` + padding) fields.
+const CONNECT_OFFSET: usize = 0;
+const READ_OFFSET: usize = CONNECT_OFFSET + size_of::<Overlapped>(); // `connect` fields.
 const WRITE_OFFSET: usize = READ_OFFSET + size_of::<Overlapped>(); // `read` fields.
 
 #[test]