Why don't the adjacent types UD2x18 and SD1x18 have helper functions, e.g. add?

[PaulRBerg]

@apuigsech asked me this question privately (via Telegram). I have posted it here because it is a good question that deserves a discussion in the public sphere (so that others can chime in to cast their support for this feature request).

The answer is - I just didn't have time to integrate helper functions into the adjacent types UD2x18 and SD1x18. PRs welcome!

The idea is to add a new file in src/ud2x18 called Helpers.sol, which would implement basic mathematical operations such as add, sub, etc.