MSRV policy

[parasyte]

I ran into an issue with an old version of the compiler installed on Linux which caused an arcane error message to appear. And it took an unusually long time to diagnose and fix the error (update the compiler).

Repro steps:

$ rustup toolchain install 1.59.0

$ cargo +1.59.0 build
    Updating crates.io index
error: failed to select a version for the requirement `zbus = "^3.0"`
candidate versions found which didn't match: 2.3.2, 2.3.1, 2.3.0, ...
location searched: crates.io index
required by package `dark-light v0.2.3 (/home/jay/other-projects/rust-dark-light)`

zbus 3 requires at least Rust 1.60.0. What is your position on a Minimum Supported Rust Version (MSRV) policy? I don't see any such policy in the repo right now.

[frewsxcv]

We should add one, and it should probably be 1.60.0 if that's what zbus requires

[Be-ing]

I've seen other Rust projects have CI jobs for both the MSRV and stable Rust.