Removing Deserializer trait

And renaming the `Serializer` trait `FastFieldCodec`.

fastfield_codecs/benches/bench.rs

@@ -39,10 +39,11 @@ mod tests {
             sum
         });
     }
-    fn bench_create<S: FastFieldCodec>(b: &mut Bencher, data: &[u64]) {
-        let mut bytes = vec![];
+    fn bench_create<Codec: FastFieldCodec>(b: &mut Bencher, data: &[u64]) {
+        let mut bytes = Vec::new();
         b.iter(|| {
-            S::serialize(&mut bytes, &data).unwrap();
+            bytes.clear();
+            Codec::serialize(&mut bytes, &data).unwrap();
         });
     }