Use constructor promotion

@benjifisher agreed, and I've already used that coding practice in the initial migration from ECA. The only 3 classes where this isn't the case yet are those that extend core classes. And there, I guess it can't be done.

Could you please have a look at the current dev release if you're happy with what we have there?

I feel a little guilty about pushing changes without any testing at all, but that is all I have time to do right now. At least, we will see whether GitLab CI finds any problems with my changes.

Those look great, thanks @benjifisher for providing them. I've just reviewed and merged them.