Remove allocations by serialize (#29)

* Changed `expand_message_xmd` input to `Iterator`

* Changed `hash_to_scalar` input to `Iterator`

* Fix rustfmt

* Add documentation for private helper types

* Fix documentation

* Improve `chain!()` syntax bias

* Move helper functions to `mod util`

src/group/mod.rs

@@ -57,8 +57,13 @@ pub trait Group:
         <D as Add<U1>>::Output: ArrayLength<u8>;
 
     /// Hashes a slice of pseudo-random bytes to a scalar
-    fn hash_to_scalar<H: BlockInput + Digest, D: ArrayLength<u8> + Add<U1>>(
-        input: &[u8],
+    fn hash_to_scalar<
+        'a,
+        H: BlockInput + Digest,
+        D: ArrayLength<u8> + Add<U1>,
+        I: IntoIterator<Item = &'a [u8]>,
+    >(
+        input: I,
         dst: GenericArray<u8, D>,
     ) -> Result<Self::Scalar, InternalError>
     where