Add defensive_assert! macro

[ggwpez]

• Change the defensive! to use expr instead of tt and allow a trailing comma.
• Add a defensive_assert! macro for convenience. Example:

use frame_support::defensive_assert;

std::panic::catch_unwind(|| {
    defensive_assert!(1 == 0, "Must fail")
}).unwrap_err();

I am not sure how diligently we use the note_worthy label, so I will put it there for now.

[ggwpez]

bot merge