Add messages to some todo items

src/bigint.rs

@@ -243,7 +243,7 @@ impl BigInt {
             let (res, overflowed) = a.overflowing_mul(b);
 
             if overflowed {
-                todo!()
+                todo!("Multiplication overflow not yet implemented")
             } else {
                 let (res, overflowed) = res.overflowing_add(carry);
 
@@ -372,7 +372,7 @@ impl Div for BigInt {
 
     #[must_use]
     fn div(self, _rhs: Self) -> Self::Output {
-        todo!()
+        todo!("Division is not yet implemented")
     }
 }
 
@@ -381,7 +381,7 @@ impl Rem for BigInt {
 
     #[must_use]
     fn rem(self, _rhs: Self) -> Self::Output {
-        todo!()
+        todo!("Modulus is not yet implemented")
     }
 }