Strip down allocation in serialization of R1CSProof (#298)

Add `InnerProductProof::to_bytes_iter`.

This semantically equivalent to `proof.to_bytes().into_iter()` except:

1. It borrows the proof;
2. It doesn't allocate

src/inner_product_proof.rs

@@ -349,6 +349,21 @@ impl InnerProductProof {
         buf
     }
 
+    /// Converts the proof into a byte iterator over serialized view of the proof.
+    /// The layout of the inner product proof is:
+    /// * \\(n\\) pairs of compressed Ristretto points \\(L_0, R_0 \dots, L_{n-1}, R_{n-1}\\),
+    /// * two scalars \\(a, b\\).
+    #[inline]
+    pub(crate) fn to_bytes_iter(&self) -> impl Iterator<Item = u8> + '_ {
+        self.L_vec
+            .iter()
+            .zip(self.R_vec.iter())
+            .flat_map(|(l, r)| l.as_bytes().iter().chain(r.as_bytes()))
+            .chain(self.a.as_bytes())
+            .chain(self.b.as_bytes())
+            .copied()
+    }
+
     /// Deserializes the proof from a byte slice.
     /// Returns an error in the following cases:
     /// * the slice does not have \\(2n+2\\) 32-byte elements,

src/r1cs/proof.rs

@@ -104,8 +104,7 @@ impl R1CSProof {
         buf.extend_from_slice(self.t_x.as_bytes());
         buf.extend_from_slice(self.t_x_blinding.as_bytes());
         buf.extend_from_slice(self.e_blinding.as_bytes());
-        // XXX this costs an extra alloc
-        buf.extend_from_slice(self.ipp_proof.to_bytes().as_slice());
+        buf.extend(self.ipp_proof.to_bytes_iter());
         buf
     }
 

src/range_proof/mod.rs

@@ -494,8 +494,7 @@ impl RangeProof {
         buf.extend_from_slice(self.t_x.as_bytes());
         buf.extend_from_slice(self.t_x_blinding.as_bytes());
         buf.extend_from_slice(self.e_blinding.as_bytes());
-        // XXX this costs an extra alloc
-        buf.extend_from_slice(self.ipp_proof.to_bytes().as_slice());
+        buf.extend(self.ipp_proof.to_bytes_iter());
         buf
     }