Add lint for panic!("{}")

[m-ou-se]

This adds a lint that warns about panic!("{}").

panic!(msg) invocations with a single argument use their argument as panic payload literally, without using it as a format string. The same holds for assert!(expr, msg).

This lints checks if msg is a string literal (after expansion), and warns in case it contained braces. It suggests to insert "{}", to use the message literally, or to add arguments to use it as a format string.

This lint is also a good starting point for adding warnings about panic!(not_a_string) later, once panic_any() becomes a stable alternative.