As of #335, TokenStream is Rc<Vec<TokenTree>>. This makes twice as many allocations as it should, because there's one that holds {strong count, weak count, data ptr, length, capacity} and a different one that holds the vector elements.

Let's look into putting these all into one allocation (except omit weak count, which we do not use) and see how it affects cargo bench --bench file --features full,parsing,test in syn.