Removing explicit uses of + mode

This removes most explicit uses of the + argument mode. Pending a
snapshot, I had to remove the forbid(deprecated_modes) pragma from
a bunch of files. I'll put it back!

+ mode still has to be used in a few places for functions that get
moved (see task.rs)

The changes outside core and std are due to the to_bytes trait and
making the compiler (with legacy modes on) agree with the libraries
(with legacy modes off) about modes.

src/test/bench/graph500-bfs.rs

@@ -251,7 +251,7 @@ fn pbfs(&&graph: arc::ARC<graph>, key: node_id) -> bfs_result {
         colors = do par::mapi_factory(*color_vec) {
             let colors = arc::clone(&color);
             let graph = arc::clone(&graph);
-            fn~(+i: uint, +c: color) -> color {
+            fn~(i: uint, c: color) -> color {
                 let c : color = c;
                 let colors = arc::get(&colors);
                 let graph = arc::get(&graph);