change requirement to rust stable

[CapSel]

Rust 1.51 implements const generics - https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html

[slightlyoutofphase]

I'd love to, eventually. However, this crate does rely on a significant number of nightly-only features that aren't actually a part of the recent stabilization. You can see all the flags I have to use here.