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/libcore/stackwalk.rs

@@ -1,5 +1,7 @@
 #[doc(hidden)]; // FIXME #3538
 
+#[legacy_modes]; // tjc: remove after snapshot
+
 // NB: transitionary, de-mode-ing.
 // XXX: Can't do this because frame_address needs a deprecated mode.
 //#[forbid(deprecated_mode)];