Add trait for saturating arithmetic (#15812)

* Add SaturatingArithmetic trait

* Use Duration saturating arithmetic

* Use new macro to fix poh_config

sdk/src/arithmetic.rs

@@ -0,0 +1,45 @@
+use std::time::Duration;
+
+/// A helper trait for primitive types that do not yet implement saturating arithmetic methods
+pub trait SaturatingArithmetic<T> {
+    fn sol_saturating_add(&self, rhs: Self) -> Self;
+    fn sol_saturating_sub(&self, rhs: Self) -> Self;
+    fn sol_saturating_mul(&self, rhs: T) -> Self;
+}
+
+/// Saturating arithmetic for Duration, until Rust support moves from nightly to stable
+/// Duration::MAX is constructed manually, as Duration consts are not yet stable either.
+impl SaturatingArithmetic<u32> for Duration {
+    fn sol_saturating_add(&self, rhs: Self) -> Self {
+        self.checked_add(rhs)
+            .unwrap_or_else(|| Self::new(u64::MAX, 1_000_000_000u32.saturating_sub(1)))
+    }
+    fn sol_saturating_sub(&self, rhs: Self) -> Self {
+        self.checked_sub(rhs).unwrap_or_else(|| Self::new(0, 0))
+    }
+    fn sol_saturating_mul(&self, rhs: u32) -> Self {
+        self.checked_mul(rhs)
+            .unwrap_or_else(|| Self::new(u64::MAX, 1_000_000_000u32.saturating_sub(1)))
+    }
+}
+
+#[cfg(test)]
+pub mod tests {
+    use super::*;
+
+    #[test]
+    fn test_duration() {
+        let empty_duration = Duration::new(0, 0);
+        let max_duration = Duration::new(u64::MAX, 1_000_000_000 - 1);
+        let duration = Duration::new(u64::MAX, 0);
+
+        let add = duration.sol_saturating_add(duration);
+        assert_eq!(add, max_duration);
+
+        let sub = duration.sol_saturating_sub(max_duration);
+        assert_eq!(sub, empty_duration);
+
+        let mult = duration.sol_saturating_mul(u32::MAX);
+        assert_eq!(mult, max_duration);
+    }
+}