Introduce enum structs to replicate TLS message format

This PR favors native coercions of enums to numerical types to help ser/de operations.
Small rearrangement of the serialization module.

src/lib.rs

@@ -416,7 +416,6 @@ pub mod opaque;
 pub mod ciphersuite;
 mod envelope;
 pub mod hash;
-mod serialization;
 
 mod elligator;
 pub mod group;
@@ -433,5 +432,7 @@ mod oprf;
 
 pub mod slow_hash;
 
+mod serialization;
+
 #[cfg(test)]
 mod tests;