Add BoxedResidue::new_with_arc (#407)

When the `std` feature is available, allows passing `BoxedResidueParams`
as an `Arc` which avoids unnecessary cloning.

src/modular/boxed_residue.rs

@@ -153,24 +153,26 @@ impl BoxedResidue {
     /// Instantiates a new [`BoxedResidue`] that represents an integer modulo the provided params.
     pub fn new(mut integer: BoxedUint, residue_params: BoxedResidueParams) -> Self {
         debug_assert_eq!(integer.bits_precision(), residue_params.bits_precision());
+        convert_to_montgomery(&mut integer, &residue_params);
 
-        let mut product = integer.mul(&residue_params.r2);
-        montgomery_reduction_boxed_mut(
-            &mut product,
-            &residue_params.modulus,
-            residue_params.mod_neg_inv,
-            &mut integer,
-        );
-
-        #[cfg(feature = "zeroize")]
-        product.zeroize();
-
+        #[allow(clippy::useless_conversion)]
         Self {
             montgomery_form: integer,
             residue_params: residue_params.into(),
         }
     }
 
+    /// Instantiates a new [`BoxedResidue`] that represents an integer modulo the provided params.
+    #[cfg(feature = "std")]
+    pub fn new_with_arc(mut integer: BoxedUint, residue_params: Arc<BoxedResidueParams>) -> Self {
+        debug_assert_eq!(integer.bits_precision(), residue_params.bits_precision());
+        convert_to_montgomery(&mut integer, &residue_params);
+        Self {
+            montgomery_form: integer,
+            residue_params,
+        }
+    }
+
     /// Bits of precision in the modulus.
     pub fn bits_precision(&self) -> u32 {
         self.residue_params.bits_precision()
@@ -240,6 +242,21 @@ impl Retrieve for BoxedResidue {
     }
 }
 
+/// Convert the given integer into the Montgomery domain.
+#[inline]
+fn convert_to_montgomery(integer: &mut BoxedUint, residue_params: &BoxedResidueParams) {
+    let mut product = integer.mul(&residue_params.r2);
+    montgomery_reduction_boxed_mut(
+        &mut product,
+        &residue_params.modulus,
+        residue_params.mod_neg_inv,
+        integer,
+    );
+
+    #[cfg(feature = "zeroize")]
+    product.zeroize();
+}
+
 #[cfg(test)]
 mod tests {
     use super::{BoxedResidueParams, BoxedUint};