Definitively decide between _opt and _exn funcs

[NatKarmios]

From #134

We frequently have two functions that do the same thing, but one returns an option, while the other fails where None would be returned.

There are two conventions for naming these:

• f returns x option, f_exn returns x or fails
• f returns x or fails, f_opt returns x option

We currently mix these - we should make this consistent.
I think I'd prefer the first approach, but we can have that conversation before implementing.