Update to use newer Rust features.

This re-introduces several cleanups that we previously deferred for not
supporting Rust 1.25.

lib/codegen/src/timing.rs

@@ -154,7 +154,7 @@ mod details {
                 fn fmtdur(mut dur: Duration, f: &mut fmt::Formatter) -> fmt::Result {
                     // Round to nearest ms by adding 500us.
                     dur += Duration::new(0, 500_000);
-                    let ms = dur.subsec_nanos() / 1_000_000;
+                    let ms = dur.subsec_millis();
                     write!(f, "{:4}.{:03} ", dur.as_secs(), ms)
                 }