Provide optional features on tokio crate (#808)

Disabling all features means the only dependency is `futures`.

Relevant pieces of the API can then be enabled with the following features:

- `codec`
- `fs`
- `io`
- `reactor`
- `tcp`
- `timer`
- `udp`
- `uds`

This also introduces the beginnings of enabling only certain pieces of the `Runtime`. As a start, the entire default runtime API is enabled via the `rt-full` feature.

src/util/future.rs

@@ -1,9 +1,12 @@
+#[cfg(feature = "timer")]
 #[allow(deprecated)]
 use tokio_timer::Deadline;
+#[cfg(feature = "timer")]
 use tokio_timer::Timeout;
 
 use futures::Future;
 
+#[cfg(feature = "timer")]
 use std::time::{Instant, Duration};
 
 
@@ -55,12 +58,14 @@ pub trait FutureExt: Future {
     /// tokio::run(future);
     /// # }
     /// ```
+    #[cfg(feature = "timer")]
     fn timeout(self, timeout: Duration) -> Timeout<Self>
     where Self: Sized,
     {
         Timeout::new(self, timeout)
     }
 
+    #[cfg(feature = "timer")]
     #[deprecated(since = "0.1.8", note = "use `timeout` instead")]
     #[allow(deprecated)]
     #[doc(hidden)]
@@ -78,6 +83,7 @@ mod test {
     use super::*;
     use prelude::future;
 
+    #[cfg(feature = "timer")]
     #[test]
     fn timeout_polls_at_least_once() {
         let base_future = future::result::<(), ()>(Ok(()));

src/util/stream.rs

@@ -1,3 +1,4 @@
+#[cfg(feature = "timer")]
 use tokio_timer::{
     throttle::Throttle,
     Timeout,
@@ -5,6 +6,7 @@ use tokio_timer::{
 
 use futures::Stream;
 
+#[cfg(feature = "timer")]
 use std::time::Duration;
 
 
@@ -25,6 +27,7 @@ pub trait StreamExt: Stream {
     /// Throttle down the stream by enforcing a fixed delay between items.
     ///
     /// Errors are also delayed.
+    #[cfg(feature = "timer")]
     fn throttle(self, duration: Duration) -> Throttle<Self>
     where Self: Sized
     {
@@ -63,6 +66,7 @@ pub trait StreamExt: Stream {
     /// tokio::run(stream);
     /// # }
     /// ```
+    #[cfg(feature = "timer")]
     fn timeout(self, timeout: Duration) -> Timeout<Self>
     where Self: Sized,
     {