Check crate MSRVs

[fuzzypixelz]

This add a check for the MSRV of all crates in the workspace. Each crate has its own MSRV specified through package.rust-version.

[Mallets]

@fuzzypixelz what's the status of this PR? Is it ready for review?

[fuzzypixelz]

@fuzzypixelz what's the status of this PR? Is it ready for review?

Right now it seems to report incorrect MSRVs. I would have to investigate that first.

[fuzzypixelz]

The issue wasn't that the job was reporting incorrect MSRVs. But that the MSRV of the crate zenohd depends on the target platform it's compiled for.

On the ubuntu-latest runner (i.e. x86_64-unknown-linux-gnu) the MSRV is 1.70. But on my Apple Silicon machine (i.e. aarch64-apple-darwin) the MSRV is 1.72.

This MSRV business is more complicated than I first thought...

[fuzzypixelz]

The problem now is handling platform-specific MSRVs. Right now we check that the rust-version field is equal to the MSRV.

If crate C1 on platform P1 has an MSRV of V1 and an MSRV of V2 on platform P2. Then the checks on platforms P1 and P2 cannot both succeed.

We cannot tolerate a rust-version lower than the MSRV (here MSRV means the minimum version that builds). So we should instead tolerate rust-versions higher than the MSRV and emit a warning for good measure.

[fuzzypixelz]

The other problem is that cargo-msrv fails to install toolchains on windows-latest with error:

error: could not create link from 'C:\\Users\\runneradmin\\.cargo\\bin\\rustup.exe' to 'C:\\Users\\runneradmin\\.cargo\\bin\\cargo.exe'

It seemingly cannot create a (symbolic? hard?) link. This is very odd since the check job installs the Stable toolchain successfully on windows-latest.