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/lib.rs

@@ -425,6 +425,8 @@ extern crate alloc;
 
 #[macro_use]
 mod impls;
+#[macro_use]
+mod util;
 pub mod errors;
 pub mod group;
 mod serialization;