libcore: update bytes/str reader to work with regions.

This optimizes away a copy.

src/test/run-pass/issue-2904.rs

@@ -69,7 +69,7 @@ mod test {
     #[test]
     fn read_simple_board() {
         let s = #include_str("./maps/contest1.map");
-        read_board_grid(io::str_reader(s));
+        io::with_str_reader(s, read_board_grid)
     }
 }