Add BoxedMontyForm::as_montgomery_mut (#1014)

This PR adds a mutable access method for the underlying Montgomery form
`BoxedUint` in `BoxedMontyForm` type.

Author: ElijahVlasov

src/modular/boxed_monty_form.rs

@@ -242,6 +242,11 @@ impl BoxedMontyForm {
         &self.montgomery_form
     }
 
+    /// Mutably access the [`BoxedMontyForm`] value in Montgomery form.
+    pub fn as_montgomery_mut(&mut self) -> &mut BoxedUint {
+        &mut self.montgomery_form
+    }
+
     /// Create a [`BoxedMontyForm`] from a value in Montgomery form.
     pub fn from_montgomery(integer: BoxedUint, params: BoxedMontyParams) -> Self {
         debug_assert_eq!(integer.bits_precision(), params.bits_precision());
@@ -375,4 +380,18 @@ mod tests {
         assert_eq!(zero.div_by_2(), zero);
         assert_eq!(one.div_by_2().mul(&two), one);
     }
+
+    #[test]
+    fn as_montgomery_mut() {
+        let modulus = Odd::new(BoxedUint::from(9u8)).unwrap();
+        let params = BoxedMontyParams::new(modulus);
+        let one = BoxedMontyForm::one(params.clone());
+        let two = one.add(&one);
+        let four = two.mul(&two);
+        let mut x = two.clone();
+
+        *x.as_montgomery_mut() = four.as_montgomery().clone();
+
+        assert_eq!(x, four);
+    }
 }