CurrentLoop: use a better justified measurement algorithm

'course the best way to justify it is with tests: hopefully those will
come shortly.

crates/cross/src/real.rs

@@ -107,6 +107,10 @@ pub trait Real:
         self == Self::zero()
     }
 
+    fn inv(self) -> Self {
+        Self::one() / self
+    }
+
     fn zero() -> Self;
     fn one() -> Self;
     fn two() -> Self;