refactor: change std::time::Duration to std::core::Duration

One step closer to `no_std`.

Signed-off-by: Stephen Waits <steve@waits.net>

benches/million-updates.rs

@@ -1,4 +1,4 @@
-use std::time::Duration;
+use core::time::Duration;
 
 use criterion::{criterion_group, criterion_main, Criterion};
 use vivaldi_nc::{NetworkCoordinate2D, NetworkCoordinate3D};

src/network_coordinate.rs

@@ -2,7 +2,7 @@
 //!
 //! For usage explanation and examples, please see the main [`crate`] documentation.
 
-use std::time::Duration;
+use core::time::Duration;
 
 use serde::{Deserialize, Serialize};
 
@@ -146,7 +146,7 @@ impl<const N: usize> NetworkCoordinate<N> {
     /// # Example
     ///
     /// ```
-    /// use std::time::Duration;
+    /// use core::time::Duration;
     /// use vivaldi_nc::NetworkCoordinate;
     ///
     /// // We always have our own NC: