path2: Extract posix/windows into their own files

Move PosixPath into posix::Path.

src/libstd/path2/windows.rs

+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Windows file path handling
+
+/// The standard path separator character
+pub static sep: u8 = '\\' as u8;
+/// The alternative path separator character
+pub static sep2: u8 = '/' as u8;
+
+/// Returns whether the given byte is a path separator
+#[inline]
+pub fn is_sep(u: &u8) -> bool {
+    *u == sep || *u == sep2
+}