Minimum Supported Rust Version

[koivunej]

In #696 I included 58835a5 which updates the README not to include any MSRV.

MSRV first appeared in c4a5364. After the adding of MSRV the version has been bumped at least for 1.45 defining the f64 -> u64 conversion in #654 and 1.46 for sized-chunks in #689. Meanwhile we've been developing and CI has always been testing and running clippy on the latest stable. Now #696 with the new lint fixes pushes it the minimum to 1.51.

Since this crate depends, even directly, on many crates which are expected to evolve in the near future thanks to const generics, I think it's best to keep only the support for latest stable.

Opening this issue up for discussion since I do not want to block #696 on the README update (which is overdue either way).