Integrate timers with runtime. (#266)

This patch integrate the new timer implementation with the runtime by
initializing a timer per worker thread. This allows minimizing the
amount of synchronization needed for using timers.

src/timer.rs

@@ -0,0 +1,85 @@
+//! Utilities for tracking time.
+//!
+//! This module provides a number of types for executing code after a set period
+//! of time.
+//!
+//! * [`Sleep`][Sleep] is a future that does no work and completes at a specific `Instant`
+//!   in time.
+//!
+//! * [`Interval`][Interval] is a stream yielding a value at a fixed period. It
+//!   is initialized with a `Duration` and repeatedly yields each time the
+//!   duration elapses.
+//!
+//! * [`Deadline`][Deadline] wraps a future, requiring that it completes before
+//!   a specified `Instant` in time. If the future does not complete in time,
+//!   then it is canceled and an error is returned.
+//!
+//! These types are sufficient for handling a large number of scenarios
+//! involving time.
+//!
+//! These types must be used from within the context of the
+//! [`Runtime`][runtime] or a timer context must be setup explicitly. See the
+//! [`tokio-timer`][tokio-timer] crate for more details on how to setup a timer
+//! context.
+//!
+//! # Examples
+//!
+//! Wait 100ms and print "Hello World!"
+//!
+//! ```
+//! use tokio::prelude::*;
+//! use tokio::timer::Sleep;
+//!
+//! use std::time::{Duration, Instant};
+//!
+//! let when = Instant::now() + Duration::from_millis(100);
+//!
+//! tokio::run({
+//!     Sleep::new(when)
+//!         .map_err(|e| panic!("timer failed; err={:?}", e))
+//!         .and_then(|_| {
+//!             println!("Hello world!");
+//!             Ok(())
+//!         })
+//! })
+//! ```
+//!
+//! Require that an operation takes no more than 300ms. Note that this uses the
+//! [`deadline`][ext] function on the [`FutureExt`][ext] trait. This trait is
+//! included in the prelude.
+//!
+//! ```
+//! # extern crate futures;
+//! # extern crate tokio;
+//! use tokio::prelude::*;
+//!
+//! use std::time::{Duration, Instant};
+//!
+//! fn long_op() -> Box<Future<Item = (), Error = ()> + Send> {
+//!     // ...
+//! # Box::new(futures::future::ok(()))
+//! }
+//!
+//! # fn main() {
+//! let when = Instant::now()  + Duration::from_millis(300);
+//!
+//! tokio::run({
+//!     long_op()
+//!         .deadline(when)
+//!         .map_err(|e| {
+//!             println!("operation timed out");
+//!         })
+//! })
+//! # }
+//! ```
+//!
+//! [runtime]: ../runtime/struct.Runtime.html
+//! [tokio-timer]: https://docs.rs/tokio-timer
+//! [ext]: ../util/trait.FutureExt.html#method.deadline
+
+pub use tokio_timer::{
+    Deadline,
+    DeadlineError,
+    Interval,
+    Sleep,
+};