libstd: Limit Duration range to i64 milliseconds.

This enables `num_milliseconds` to return an `i64` again instead of
`Option<i64>`, because it is guaranteed not to overflow.

The Duration range is now rougly 300e6 years (positive and negative),
whereas it was 300e9 years previously. To put these numbers in
perspective, 300e9 years is about 21 times the age of the universe
(according to Wolfram|Alpha). 300e6 years is about 1/15 of the age of
the earth (according to Wolfram|Alpha).

src/libstd/io/net/unix.rs

@@ -66,21 +66,15 @@ impl UnixStream {
     ///
     /// If a `timeout` with zero or negative duration is specified then
     /// the function returns `Err`, with the error kind set to `TimedOut`.
-    /// If the timeout is larger than 2^63 milliseconds, the function also
-    /// returns `Err` with the error kind set to `TimedOut`.
     #[experimental = "the timeout argument is likely to change types"]
     pub fn connect_timeout<P: ToCStr>(path: &P,
                                       timeout: Duration) -> IoResult<UnixStream> {
         if timeout <= Duration::milliseconds(0) {
             return Err(standard_error(TimedOut));
         }
-        let timeout_ms = timeout.num_milliseconds().map(|x| { x as u64 });
-        if timeout_ms.is_none() {
-            return Err(standard_error(TimedOut));
-        }
 
         LocalIo::maybe_raise(|io| {
-            let s = io.unix_connect(&path.to_c_str(), timeout_ms);
+            let s = io.unix_connect(&path.to_c_str(), Some(timeout.num_milliseconds() as u64));
             s.map(|p| UnixStream { obj: p })
         }).map_err(IoError::from_rtio_error)
     }