Define MSRV policy

[anp]

Need to define a project policy for the minimum Rust version we support.

CI still needs to use a pinned nightly version for collecting coverage, but once #221 happens that'll be the only obstacle between us and all-stable-all-the-time.

My initial thought is picking something like "current stable release and previous stable release".

[zetanumbers]

Previously mox used proc_macro_hack crate which:

Note: As of Rust 1.45 this crate is superseded by native support for #[proc_macro] in expression position. Only consider using this crate if you care about supporting compilers between 1.31 and 1.45.

EDIT: I guess this is important