Serialize the binder's annotations into a byte string.

def to_bytes(self):
        """Serialize the binder's annotations into a byte string."""
        for tokens in self.tokens:
            assert len(tokens.shape) == 2, tokens.shape
        lengths = [len(tokens) for tokens in self.tokens]
        msg = {
            "attrs": self.attrs,
            "tokens": numpy.vstack(self.tokens).tobytes("C"),
            "spaces": numpy.vstack(self.spaces).tobytes("C"),
            "lengths": numpy.asarray(lengths, dtype="int32").tobytes("C"),
            "strings": list(self.strings),
        }
        return gzip.compress(srsly.msgpack_dumps(msg))