Don't allow for multiplying unit types by themselves

The result should actually be the unit type squared, but as this is something we don't have to worry about, let's just disallow it.

Author: alexdewar

src/units.rs

@@ -2,7 +2,7 @@
 
 use serde::{Deserialize, Serialize};
 use std::fmt;
-use std::ops::{AddAssign, SubAssign};
+use std::ops::{AddAssign, Mul, SubAssign};
 
 macro_rules! base_unit_struct {
     ($name:ident) => {
@@ -20,12 +20,6 @@ macro_rules! base_unit_struct {
         )]
         pub struct $name(pub f64);
 
-        impl std::ops::Mul<$name> for $name {
-            type Output = $name;
-            fn mul(self, rhs: $name) -> $name {
-                $name(self.0 * rhs.0)
-            }
-        }
         impl std::ops::Div<$name> for $name {
             type Output = Dimensionless;
             fn div(self, rhs: $name) -> Dimensionless {
@@ -94,6 +88,14 @@ impl From<Dimensionless> for f64 {
     }
 }
 
+impl Mul for Dimensionless {
+    type Output = Dimensionless;
+
+    fn mul(self, rhs: Self) -> Self::Output {
+        Dimensionless(self.0 * rhs.0)
+    }
+}
+
 impl Dimensionless {
     /// Raises this dimensionless number to the power of `rhs`.
     pub fn powi(self, rhs: i32) -> Self {