Schema changes as minor version bumps

[guybedford]

It might be time to consider making schema changes minor version bumps, and then trying to make minors non-breaking as much as possible otherwise.

Thoughts?

[drewcrawford]

I would generally be more in favor of stable schema as a goal.

I am also aware that 0.2.106 broke schema with main four days later (#4788). In general if I have any concern about stability it is less about breakage between released versions and more about the breakage between the last released version and main, because in practice, I need to live around main at present.

This may also be influenced by the fact that if you're targeting +atomics you have to be on nightly, and so I generally assume a certain amount of breakage on a regular basis.