Fix breaking introduction of variable in Semiring.Primality

[MatthewDaggitt]

Fixes accidental breaking change in #2772.

[jamesmckinna]

For the sake of v2.3, I'm happy to revert a prior change.

But the general principle of avoiding implicits in favour of variables is still a good one, and will, from time to time, introduce breaking changes because of re-ordering of telescopic bindings.

User code which relies on explicit instantiation of implicit arguments is inherently brittle, unless this points to the need to make the argument explicit, which should be flagged as a separate issue?

I hope that for v3.0 we can make all of this more systematic?

[MatthewDaggitt]

I agree with all your points, particularly about how using implicits widely is very fragile! But I do think that we should be careful in introducing variables in minor version bumps and try to prioritise making breaking ones in major versions bumps nonetheless.