`syn::Punctuated` lacks an indexing method that returns an `Option<T>`

[KiChjang]

Per title.

index and index_mut doesn't fit the bill as both of them do not (and should not) return an Option, however it still makes sense to have an API that, given an index, would return an Option<T> that indicates either it does contain a T at the given index, or that the index is out of bounds and a None is returned.