feature request: add support save multiple msg into same MsgSaver.

[whunmr]

like following case:

MsgSaver<MsgPayloadRsp> bob_to_alice_rsp;
MsgPayload payload;

    msg_interaction(
            alice ---->bob(EV_ALICE_REQ, &payload);
            alice<<----bob(EV_BOB_RSP             , ___save_to(bob_to_alice_rsp));
            alice<<----bob(EV_BOB_RSP             , ___save_to(bob_to_alice_rsp));
            alice<<----bob(EV_BOB_RSP             , ___save_to(bob_to_alice_rsp));
            alice<<----bob(EV_BOB_RSP             , ___save_to(bob_to_alice_rsp));
    );

and using [] to access saved values like bob_to_alice_rsp[0] to access first saved message.