offer! macro does not catch too-small arity

[plaidfinch]

This compiles, but it shouldn't:

use dialectic::prelude::*;

type S = Session! {
    offer {
        0 => {}
    }
};

#[Transmitter(Tx)]
#[Receiver(Rx)]
async fn run<Tx, Rx, E>(chan: Chan<S, Tx, Rx>) -> Result<(), E>
where
    E: From<Tx::Error> + From<Rx::Error>,
{
    offer!(in chan {
        // no branches!!!
    })?
}

The current offer! macro (not on the carrier-type branch) does not catch this error, and indeed can't catch this error in the macro itself. This bug increases the urgency with which I would like to merge the carrier-type branch, because the offer! macro which integrates Vesta should catch this bug.