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

[programmerjake]

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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.