Deprecate std::sync::TaskPool

Rather than stabilize on the current API, we're going to punt this
concern to crates.io, to allow for faster iteration.

If you need this functionality, you might look at https://github.com/carllerche/syncbox

[breaking-change]

Author: aturon

src/libstd/sync/task_pool.rs

@@ -10,11 +10,11 @@
 
 //! Abstraction of a thread pool for basic parallelism.
 
-#![unstable(feature = "std_misc",
-            reason = "the semantics of a failing task and whether a thread is \
-                      re-attached to a thread pool are somewhat unclear, and the \
-                      utility of this type in `std::sync` is questionable with \
-                      respect to the jobs of other primitives")]
+#![deprecated(since = "1.0.0",
+              reason = "This kind of API needs some time to bake in \
+                        crates.io. Consider trying \
+                        https://github.com/carllerche/syncbox")]
+#![unstable(feature = "std_misc")]
 
 use core::prelude::*;