Merge #1209

1209: Time: Add from_hz function for Duration. r=Dirbaio a=CBJamo

I found myself doing things like this

```rust
    let rate_us = 1_000_000 / rate_hz;
    let mut ticker = Ticker::every(Duration::from_micros(rate_us));
```

Several times, and figured it was worth adding a little convenience function to handle that. This also makes the calculation const, which is a nice little upside. The compiler might have been doing that already, but this makes sure. 

Speaking of const, would it be better to give hz as a float? Obviously we'd want to avoid that at runtime since many targets don't have a fpu, but if it's at compile time that doesn't matter and a float may be more ergonomic.

Co-authored-by: Caleb Jamison <caleb@cbjamo.com>

embassy-time/src/duration.rs

@@ -81,6 +81,20 @@ impl Duration {
         }
     }
 
+    /// Creates a duration corresponding to the specified Hz.
+    /// NOTE: Giving this function a hz >= the TICK_HZ of your platform will clamp the Duration to 1
+    /// tick. Doing so will not deadlock, but will certainly not produce the desired output.
+    pub const fn from_hz(hz: u64) -> Duration {
+        let ticks = {
+            if hz >= TICK_HZ {
+                1
+            } else {
+                (TICK_HZ + hz / 2) / hz
+            }
+        };
+        Duration { ticks }
+    }
+
     /// Adds one Duration to another, returning a new Duration or None in the event of an overflow.
     pub fn checked_add(self, rhs: Duration) -> Option<Duration> {
         self.ticks.checked_add(rhs.ticks).map(|ticks| Duration { ticks })