Get doc tests to pass without std and shrink build runs

[KodrAus]

For #611

Our build requirements explode when new features are added. What we really want is just to make sure that each feature works independently, and that they all work together. This PR does that by avoiding using the feature powerset in CI. I've also made sure our doc tests actually work when the std feature is disabled.

[rrichardson]

It would appear that the --each-feature run fails on Windows. Presumably because the env vars don't evaluate the same way in powershell.
I pulled this branch into my six-seven-eight branch and broke out the windows-2019 into separate scripts.. it seems to have worked.

[KodrAus]

Thanks for carrying this forwards, I think that's fair to unblock V6-V8 support, I'll look at reducing the duplication asynchronously.