std: Deprecate the future feature

This commit deprecates the `sync::Future` type to be developed outside in
crates.io instead.

Author: alexcrichton

src/libstd/sync/future.rs

@@ -32,6 +32,10 @@
             reason = "futures as-is have yet to be deeply reevaluated with recent \
                       core changes to Rust's synchronization story, and will likely \
                       become stable in the future but are unstable until that time")]
+#[deprecated(since = "1.2.0",
+             reason = "implementation does not match the quality of the \
+                       standard library and this will likely be prototyped \
+                       outside in crates.io first")]
 
 use core::prelude::*;
 use core::mem::replace;