path2: Replace the path module outright

Remove the old path.
Rename path2 to path.
Update all clients for the new path.

Also make some miscellaneous changes to the Path APIs to help the
adoption process.

src/libstd/unstable/dynamic_lib.rs

@@ -225,7 +225,7 @@ pub mod dl {
 
     pub unsafe fn open_external(filename: &path::Path) -> *libc::c_void {
         #[fixed_stack_segment]; #[inline(never)];
-        do os::win32::as_utf16_p(filename.to_str()) |raw_name| {
+        do os::win32::as_utf16_p(filename.as_str().unwrap()) |raw_name| {
             LoadLibraryW(raw_name)
         }
     }