feat: added a Drift trait and a "default" ConstantDrift implementation

src/drift.rs

@@ -0,0 +1,14 @@
+use std::fmt::Debug;
+
+pub trait Drift: Copy + Debug {
+    fn variance_delta(&self, elapsed: i64) -> f64;
+}
+
+#[derive(Clone, Copy, Debug)]
+pub struct ConstantDrift(pub f64);
+
+impl Drift for ConstantDrift {
+    fn variance_delta(&self, elapsed: i64) -> f64 {
+        elapsed as f64 * self.0 * self.0
+    }
+}