Remove by-copy mode from std, mostly

One instance remains in net_tcp due to a foreign fn. Lots of
instances remain in serialization.rs, but IIRC that is being removed.

I had to do unholy things to task-perf-word-count-generic to get it
to compile after demoding pipes. I may well have messed up its
performance, but it passes.

src/libstd/arc.rs

@@ -1,5 +1,5 @@
 // NB: transitionary, de-mode-ing.
-// tjc: forbid deprecated modes again after snap
+#[forbid(deprecated_mode)];
 /**
  * Concurrency-enabled mechanisms for sharing mutable and/or immutable state
  * between tasks.