Avoid linting unreachable_code on todo!()

[llogiq]

Because todo!() is often used as a placeholder while working on the code, we should avoid linting unreachable_code on code after those macro uses. Instead let's add a todo_macro_uses lint that we can selectively activate e.g. when compiling a release.

This will reduce message churn and allow users to concentrate on the warnings and errors that matter while they work on the code.

Rendered

[llogiq]

Note that rust#149543 has an implementation.