MSRV Policy

We need to commit to one at some point. I'm weighing some tradeoffs.

First, the language legitimately improves all the time. Not just new features but improvements to syntax and more.

Second though, I have an eye to ensuring we could use this in safety critical contexts where certified toolchains might matter.

As such, we may want to have an MSRV that tracks Ferrocene releases so that automotive users have certified toolchains.