(improvement) serializers: add Cython-optimized serialization for VectorType

Add cassandra/serializers.pyx and cassandra/serializers.pxd implementing
Cython-optimized serialization that mirrors the deserializers.pyx architecture.

Implements type-specialized serializers for the three subtypes commonly used
in vector columns:
- SerFloatType: 4-byte big-endian IEEE 754 float
- SerDoubleType: 8-byte big-endian double
- SerInt32Type: 4-byte big-endian signed int32

SerVectorType pre-allocates a contiguous buffer and uses C-level byte swapping
for float/double/int32 vectors, with a generic fallback for other subtypes.
GenericSerializer delegates to the Python-level cqltype.serialize() classmethod.

Factory functions find_serializer() and make_serializers() allow easy lookup
and batch creation of serializers for column types.

Benchmarks show ~30x speedup over the current io.BytesIO baseline and ~3x
speedup over Python struct.pack for Vector<float, 1536> serialization.

No setup.py changes needed - the existing cassandra/*.pyx glob already picks
up new .pyx files.

Author: mykaul

cassandra/deserializers.pyx

@@ -481,7 +481,15 @@ cpdef Deserializer find_deserializer(cqltype):
 
 
 def obj_array(list objs):
-    """Create a (Cython) array of objects given a list of objects"""
+    """Create a (Cython) array of objects given a list of objects.
+
+    Returns the plain list for empty input since ``cython_array`` does
+    not support zero-length shapes.  Callers that use
+    ``cdef Deserializer[::1]`` typed memoryviews must guard against
+    empty input before assignment.
+    """
+    if not objs:
+        return objs
     cdef object[:] arr
     cdef Py_ssize_t i
     arr = cython_array(shape=(len(objs),), itemsize=sizeof(void *), format="O")

cassandra/serializers.pxd

@@ -0,0 +1,20 @@
+# Copyright ScyllaDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+cdef class Serializer:
+    # The cqltypes._CassandraType corresponding to this serializer
+    cdef object cqltype
+
+    cpdef bytes serialize(self, object value, int protocol_version)