Define an MSRV

[MabezDev]

The readme states the MSRV is the latest stable Rust, but I think now that defmt is a staple in the embedded-rust ecosystem you should consider picking an MSRV and only bumping when needed (and subsequently bumping the version in a semver compatible way too).

[Urhengulas]

How is this handled usually? Do we build and test against the latest stable and the MSRV in CI?

subsequently bumping the version in a semver compatible way too

What do you mean by that? Would you consider it a breaking change if the MSRV is bumped?

[MabezDev]

How is this handled usually? Do we build and test against the latest stable and the MSRV in CI?

Yes.

What do you mean by that? Would you consider it a breaking change if the MSRV is bumped?

There doesn't seem to be a clear answer here, at least upstream Rust hasn't taken a stance. In general though, unless defmt needs to opt in to new features, keeping MSRV as latest STABLE seems a bit silly, especially since defmt is used so widely in the ecosystem.

[thejpster]

I suspect we'll tie the MSRV to Ferrocene's upstream equivalents.