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/cell.rs

@@ -1,4 +1,4 @@
-// tjc: forbid deprecated modes again after snap
+#[forbid(deprecated_mode)];
 /// A dynamic, mutable location.
 ///
 /// Similar to a mutable option type, but friendlier.