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};