Expand assert!(expr, args..) to include $crate for hygiene on 2021.
This makes `assert!(expr, args..)` properly hygienic in Rust 2021.
This is part of rust-lang/rfcs#3007, see #80162.
Before edition 2021, this was a breaking change, as `std::panic` and `core::panic` are different. In edition 2021 they will be identical, making it possible to apply proper hygiene here.