Bump MSRV to 1.43

[tertsdiepraam]

I'd love to see the more rewrite (#2109) get merged. Unfortunately, there seems to be a bug in crossterm that was fixed in a version that is only compatible with Rust 1.43.0 (see #2109 (comment)). Therefore, I think we should bump the MSRV to 1.43.0.

Other drivers:

• Using attributes on if expressions is nice for platform-specific code.
• Rust 1.42 introduced the new Cargo.lock format, so (I think) we won't have to do cargo +1.40.0 update anymore.

[sylvestre]

sure, let's do it
we have enough good reasons

[miDeb]

Also, Rust 1.42 introduced the matches! macro. There's been more than one occasion where I wanted to use it :)