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/test.rs

@@ -5,7 +5,7 @@
 // simplest interface possible for representing and running tests
 // while providing a base that other test frameworks may build off of.
 
-#[warn(deprecated_mode)];
+#[forbid(deprecated_mode)];
 
 use core::cmp::Eq;
 use either::Either;