Implement From conversion for non-empty hardcoded arrays

[bitdriftr]

This PR adds From conversion for rust arrays of generic size. Thanks to const expressions, we can ensure the size is not zero, making the conversion infallible.

[FintanH]

This is nice, and I played around with it myself. I made some changes, but because GitHub is hostile to making new revisions to someone else's fork I had to create a new PR myself #81.

This led me to realising that constantly using stable for builds will result in clippy errors that should not matter to us, if we maintain a stable MSRV. So I created a PR for splitting out the checks #82.

However, I cannot merge this, and it seems like I'm not a maintainer on this repository, however I do have publishing rights on crates.io.

So for the time being I've pushed these changes to my fork https://github.com/FintanH/nonempty. Either @cloudhead needs to add me as maintainer or I might have to maintain the crate via my fork instead.