CI: Also run tests with `--no-default-features`

[mqudsi]

This patch configures CI to assert that cargo test --no-default-features also passes (in addition to the default combinatoric feature matrix).

An additional commit is included that fixes the existing errors preventing cargo test from passing under --no-default-features. The commit message explains the approach taken.

This PR targets 0.5.x as some of the fixes were to 0.5.x-only impls; I'll file a separate PR for 0.4.x after this one lands in case any changes are needed or requested.

I don't know if any of the CI matrix configurations will fail with the new test so I'm pushing this to initiate a CI run against it. If any fail, I'll revise the branch to address them accordingly.

[esheppa]

Thanks for this PR @mqudsi - this looks like a good complement to some work in #814, as this PR has picked up some issues with the doc tests that I missed on that PR - what do you think @djc?

[djc]

Yes, I was thinking we should rebase it on that PR (or I should just get off my ass and review it).

[djc]

@mqudsi would you mind rebasing this on current 0.4.x?

[pitdicker]

Thank you! Merged in https://github.com/chronotope/chrono/pull/1059.