auto merge of #9285 : sfackler/rust/future, r=alexcrichton

The `Drop` implementation was used to prevent `Future` from being implicitly copyable. Since `~fn`s are no longer copyable, this is no longer needed. I added a cfail test to make sure that this is actually the case.

I method-ized all of the `Future` creation methods and added a new one, `spawn_with`, which is similar to `task::spawn_with`.

I also got rid of some unused imports in tests.

src/test/compile-fail/future_not_copyable.rs

@@ -0,0 +1,19 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+extern mod extra;
+
+use extra::future::Future;
+
+fn main() {
+    let f = Future::from_value(());
+    let g = f;
+    f.unwrap(); //~ ERROR use of moved value
+}