feat: skip serialising unused field

Changes the serialised representation to skip encoding the always null
"_key_type" field, which exists only to constrain the data type the
filter encodes in code, rather than holding any meaningful state.

src/bloom.rs

@@ -192,6 +192,8 @@ where
     hasher: H,
     bitmap: B,
     key_size: FilterSize,
+
+    #[cfg_attr(feature = "serde", serde(skip))]
     _key_type: PhantomData<T>,
 }
 

tests/fixtures/compressed_bitmap.json

@@ -11,6 +11,5 @@
     ],
     "max_key": 256
   },
-  "key_size": "KeyBytes1",
-  "_key_type": null
+  "key_size": "KeyBytes1"
 }