It would be nice if there was a way to tell compilers not to drop the lock stack and thus issue warnings when accessing a symbol that needs a lock inside `Assume`/`Assert`.

Hit by (at least): @ajtowns , @jnewbery , me.
{
    BOOST_CHECK_EQUAL(
        MessageVerify(
            "invalid address",
            "signature should be irrelevant",
            "message too"),
        MessageVerificationResult::ERR_INVALID_ADDRESS);
    BOOST_CHECK_EQUAL(
        MessageVerify(
            "3B5fQsEXEaV8v6U3ejYc8XaKXAkyQj2MjV",
            "signature should be irrelevant",
            "message too"),
        MessageVerificationResult::ERR_ADDRESS_NO_KEY);
    BOOST_CHECK_EQUAL(
        MessageVerify(