feat: auto32/auto64

[henryiii]

Add auto32/auto64.

Closes #551.

[henryiii]

I've rebased on top of #545 since it looks like that one will go in. When looking at this PR, just look at the last commit. Looks like #545 is based on a slightly older master than I intended, so the CIBW_ARCHS README change isn't picked up, so I'll re-rebase when #545 is in. @joerick I've updated the docs quite a bit on CIBW_ARCHS, to be more consistent with the rest of the options, especially now that we have some useful on-all-platforms arch settings. It would be great if we could get this in as the last PR before 1.8.0.

I've also normalized a names in the tests for better consistency; could have gone into #545 but didn't realize it at the time I pushed that one.

[henryiii]

@joerick and/or @YannickJadoul Feel free to look at 7ccca9e by itself to speed up the process, as soon as #545 goes in that's what I'll rebase on master.

[joerick]

#545 is the last PR for 1.8.0, otherwise we'll never get it out! I think the docs currently on master are fine for documenting the emulation use-case, so we don't need to rush to get this in.

[joerick]

Looks good!