std: Refactor time module a bit

Put `Duration` in `time::duration`, where the two constants can
be called just `MAX` and `MIN`. Reexport from `time`.
This provides more room for the time module to expand.

src/libstd/time/mod.rs

@@ -0,0 +1,15 @@
+// Copyright 2012-2014 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.
+
+//! Temporal quantification.
+
+pub use self::duration::Duration;
+
+pub mod duration;