Add MSRV check to CI

[austinabell]

Currently, changes can bump the MSRV without noticing. Would be good to have this documented and stable for #51.

Noticed from #104 bumping MSRV without any failing CI

[matklad]

FWIW, I’ve recently written down my current thoughts on MSRV here: matklad/once_cell#201

[austinabell]

Hmm, I completely understand the viewpoint, and maybe we make no guarantees about bumping msrv during minor/patch versions, but is it not still beneficial to have this msrv controlled and visible?

Feel free to close, I don't have an actual opinion on this

[matklad]

Crates should document their MSRV policy, and, if the policy is different from "latest stable" it should be tested on CI.

I am not entirely sure what's the right MSRV policy for borsh, the following makes sense to me:

• MSRV is not breaking
• MSRV is six months at least
• explicit non-support for debian stable and such
• newer compilers can be used if required by a feature (cosnt-generics) or via feacher-checking in build.rs (eg, via rustc-version or autocfg crates).

If we have abundance of resources, 2 year MSRV would also make sense to me

If we are very short on resources, I think declaring "latest stable" won't be catastrophic either.

[dj8yfo]

taking this for trial and error and resolution