Revamp TaskBuilder API

This patch consolidates and cleans up the task spawning APIs:

* Removes the problematic `future_result` method from `std::task::TaskBuilder`,
  and adds a `try_future` that both spawns the task and returns a future
  representing its eventual result (or failure).

* Removes the public `opts` field from `TaskBuilder`, instead adding appropriate
  builder methods to configure the task.

* Adds extension traits to libgreen and libnative that add methods to
  `TaskBuilder` for spawning the task as a green or native thread.

Previously, there was no way to benefit from the `TaskBuilder` functionality and
also set the scheduler to spawn within.

With this change, all task spawning scenarios are supported through the
`TaskBuilder` interface.

Closes #3725.

[breaking-change]

src/test/run-pass/task-stderr.rs

@@ -0,0 +1,26 @@
+// Copyright 2014 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.
+
+use std::io::{ChanReader, ChanWriter};
+use std::task::build;
+
+fn main() {
+    let (tx, rx) = channel();
+    let mut reader = ChanReader::new(rx);
+    let stderr = ChanWriter::new(tx);
+
+    let res = build().stderr(box stderr as Box<Writer + Send>).try(proc() -> () {
+        fail!("Hello, world!")
+    });
+    assert!(res.is_err());
+
+    let output = reader.read_to_str().unwrap();
+    assert!(output.as_slice().contains("Hello, world!"));
+}