Improved RESULT ennum successful string

[LouisDesdoigts]

Closes #55

[patrick-kidger]

I'm not sure what's causing the test failure, as I can't reproduce it locally. (Even setting the same seed etc.) So I think we can ignore that for now.

I have however realised that this change will break Optimistix: right now, eqx.Enumerations check that you don't change a message when subclassing, and now optimistix.RESULTS.successful will try to change it back to an empty string. (As optimistix.RESULTS subclasses lineax.RESULTS.)

I'm going to downgrade that to a warning. Once the next Equinox release is out, I'll merge this.

[patrick-kidger]

Coming back around, I'm afraid that I'm going to close this as a 'wontfix'. :(
It ends up being a little bit annoying to change this, and then propagate the changed result across the ecosystem, in a way that won't spam everyone with errors and warnings. Doable, but probably not worth the hassle.

Thank you anyway!