- https://cplusplus.com/reference/iterator/iterator/
- https://www.codeproject.com/Articles/36530/An-Introduction-to-Iterator-Traits
- https://www.internalpointers.com/post/writing-custom-iterators-modern-cpp
- Capacity increases by x2 once full. eg. 1, 2, 4, 4, 8, 8...