Implementation should aim to use `core::ptr::NonNull` internally

[cmazakas]

Make sure the implementation is like std::vec::Vec where the size of Option<Vec> is the same as Vec.

[cmazakas]

Unfortunately, this can't be implemented for the MiniVec because we've no way of knowing what's a default-constructed state or not. Vec can infer it has a dangling pointer by checking the capacity().