add Fold/VisitMut methods for Vec<T>, Option<T>, Punctuated, etc.

[programmerjake]

I'm trying to build a Fold visitor that removes all Attributes that match a pattern, using Fold::fold_attribute doesn't work because I want to replace the attribute with nothing, not another attribute.

I think adding Fold::fold_attributes is a good solution.

I also ran into this problem when trying to transform Stmt into nothing, I had to construct a no-op Stmt.