Change final for mut keyword

[JorgeCepeda]

I don't like the idiom of making most variables final, adding words to the code.

Instead I'd prefer to assume variables are final unless there's a mut keyword added to it, like in other languages, and remove the final keyword altogether.

[CC007]

Having a setting that can specify final being the default for fields, parameters and local vars would indeed be nice.

You probably don't want to touch final methods and classes though, since it has a somewhat different meaning there.

[JorgeCepeda]

I agree, in the case of final classes I see it as a good option to favour composition over inheritance