[DRY] More redundant zero fields in Algebra.Structures

[jamesmckinna]

Cf. #2195 and #2253

• IsNonAssociativeRing admits enough properties to make the zero field redundant by exactly the same argument as for IsRingWithoutOne (so the redundancy is a bug, and removing it would be breaking)
• correspondingly, the argument for IsRingWithoutOne makes no use of *-assoc...

Possible remedy: introduce a further refinement in which a new IsNonAssociativeRingWithoutOne would the 'right' home for such an argument, with each of the above two structures inheriting from that...?

See for example this blob and prior refactoring via Quasiring... for a possible solution.

[jamesmckinna]

@lexvanderstoep Are you planning a PR at any point in this direction?

[lexvanderstoep]

Yes @jamesmckinna, there’s #2410 which was shelved at the time due to it being a breaking change.

What do you think? Is it a better time now?

[jamesmckinna]

@lexvanderstoep apologies! I'd completely forgotten (and had draft:false turned on while looking back over open PRs... d'oh!).
We'll be discussing a new release, and the development path after that, in early January, so let's review this then. In the meantime, can you see whether your PR builds against the current version of master? Thanks!

[lexvanderstoep]

Done! Still works like a charm.

I'll leave the PR as a draft for now then. We can come back to it when we get around to the new release :)