Support #[mutants::skip] attribute on more statements

[sourcefrog]

It would be helpful if you could put #[mutants::skip] on arbitrary statements such as const, let, or expression statements.

There are some restrictions (https://doc.rust-lang.org/reference/attributes.html) on where they can go but I think several useful cases could be handled in stable Rust.

Also, some people might have trees that are only expected to build on nightly rust, and they could use attributes freely...