time: use bit manipulation instead of modulo (#4480)

time: resolve TODO

## Motivation

Existing `TODO` comment in `src/time/driver/wheel/level.rs`.

## Solution

`level_range()` always return a strictly positive power of 2. If `b` is a
strictly positive power of 2, `a - (a % b)` is equal to `a & !(b - 1)`.

tokio/src/time/driver/wheel/level.rs

@@ -143,8 +143,9 @@ impl Level {
         let level_range = level_range(self.level);
         let slot_range = slot_range(self.level);
 
-        // TODO: This can probably be simplified w/ power of 2 math
-        let level_start = now - (now % level_range);
+        // Compute the start date of the current level by masking the low bits
+        // of `now` (`level_range` is a power of 2).
+        let level_start = now & !(level_range - 1);
         let mut deadline = level_start + slot as u64 * slot_range;
 
         if deadline <= now {