Planning the next major release

[dstansby]

#112 (merged) is quite a big change, so I would advocate for it being in a new major release. There's a few other issues that would be major breaking changes, so I thought I'd open this issue to coordinate what we want to put in the next release. In terms of strategy, I'm pro bunching together a bunch of breaking changes in one big release with clear upgrade docs, to minimise the number of times downstream users have to update their code.

Issues we could include:

• optional members #30
• GroupSpec signature #20
• are the generic types worth it #96

Any others?

[d-v-b]

I think #120 ticks these three boxes. The changes in that PR are non-breaking, but if it's easier to make them breaking, then I can shift the direction of that PR.