Enhance `ensure` macro to allow trailing commas

[Henning-K]

For clarification: Do you mean "trailing comma" as in (drawing on the example from the docs):

ensure!(user_id > 0, InvalidUser {user_id}, );
------------------------------------------^

Or that the part after the first comma can be missing, because that doesn't really make sense.

EDIT 2020-09-18: After nine days of radio silence I'll now just assume the first option of the original comment was the intended modification.

[shepmaster]

Sorry! I marked this notification as read without responding to it. Yes, the goal is to support

ensure!(
    some_long_condition,
    AnotherLongThing { more_long },
    // ---------------------------^
);