no_release_panic and more

[leonardo-m]

I think an improved version of this no_panic is worth having in the core Rust language instead of in an external crate because I think this attribute goes very well with the correctness purposes of Rust language. By improved version I meant with a cleaner error message, and perhaps working recursively (requiring a function to call other no_panic functions)?

Perhaps beside the #[no_panic] is also worth having #[no_release_panic] that tests for the presence of panics only when the function is compiled in release mode.

[dtolnay]

I would prefer not to build this into this crate.