Return the head block of the current chain.

def chain_head(self):
        """
        Return the head block of the current chain.
        """
        (vec_ptr, vec_len, vec_cap) = ffi.prepare_vec_result()

        try:
            _libexec(
                'commit_store_get_chain_head',
                self.pointer,
                ctypes.byref(vec_ptr),
                ctypes.byref(vec_len),
                ctypes.byref(vec_cap))
        except ValueError:
            return None

        return self.deserialize_block(
            ffi.from_rust_vec(vec_ptr, vec_len, vec_cap))