time: move DelayQueue to tokio-util (#2897)

This change is intended to do the minimum to unblock 0.3; as such, for now, we
duplicate the internal `time::wheel` structures in tokio-util, rather than trying
to refactor things at this stage.

Co-authored-by: Bryan Donlan <[email protected]>

tokio-util/src/time/mod.rs

@@ -0,0 +1,47 @@
+//! Additional utilities for tracking time.
+//!
+//! This module provides additional utilities for executing code after a set period
+//! of time. Currently there is only one:
+//!
+//! * `DelayQueue`: A queue where items are returned once the requested delay
+//!   has expired.
+//!
+//! This type must be used from within the context of the `Runtime`.
+
+use std::time::Duration;
+
+mod wheel;
+
+#[doc(inline)]
+pub mod delay_queue;
+
+pub use delay_queue::DelayQueue;
+
+// ===== Internal utils =====
+
+enum Round {
+    Up,
+    Down,
+}
+
+/// Convert a `Duration` to milliseconds, rounding up and saturating at
+/// `u64::MAX`.
+///
+/// The saturating is fine because `u64::MAX` milliseconds are still many
+/// million years.
+#[inline]
+fn ms(duration: Duration, round: Round) -> u64 {
+    const NANOS_PER_MILLI: u32 = 1_000_000;
+    const MILLIS_PER_SEC: u64 = 1_000;
+
+    // Round up.
+    let millis = match round {
+        Round::Up => (duration.subsec_nanos() + NANOS_PER_MILLI - 1) / NANOS_PER_MILLI,
+        Round::Down => duration.subsec_millis(),
+    };
+
+    duration
+        .as_secs()
+        .saturating_mul(MILLIS_PER_SEC)
+        .saturating_add(u64::from(millis))
+}