Local var renaming strips mut away

[aochagavia]

Before:

fn test() {
    let mut x = 42;
    let y = x;
}

After (rename x to foo):

fn test() {
    let foo = 42;
    let y = foo;
}

[aochagavia]

@kjeremy mentioned in #135 (comment) that we probably need to massage the value passed into DeclarationDescriptor::new before hand to solve that (or adjust the node passed in to that function which is more likely).