CI improvements

[fjarri]

• Various small CI tweaks (see separate commits)
• Add a "stable test" with a fixed Rust version and a lockfile. Fixes Add a "stable dependencies test" CI task  #44

Note: the docs for cargo test say about --locked: "Cargo will exit with an error when either of the following scenarios arises: ... Cargo attempted to change the lock file due to a different dependency resolution." I think it's not going to error out if some dependency bumps the patch version, but I'm not sure.

@dvdplm if you think that the stable test is not really needed, that can be reverted. I'm not sure myself.

[coveralls]

Pull Request Test Coverage Report for Build 11505804412

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.243%

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Totals
Change from base Build 11505585913:	0.0%
Covered Lines:	1204
Relevant Lines:	1690

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💛 - Coveralls

[dvdplm]

Note to self: we should mention in the README that the crate is nostd, perhaps in the "goals" section.

[dvdplm]

I wish there was an "organization-wide" way of defining this, so that when we change it, it applies to all crates.

[dvdplm]

As for the stable test: I have a (weak) preference for omitting the --locked because that means that the tests will indeed fail when deps go out of sync. I'd rather be alerted early to such things than tackling them all at once. That said, almost nobody agrees with me so I'm likely wrong!

Let's go with what the docs say for now and if it ever becomes annoying we can re-assess?