Get lightbeam compiling on stable Rust (#2370)

This will hopefully remove a small thorn in our side with periodic
nightly breakage due to nightly features changing. This commit moves
lightbeam to stable Rust, swapping out `staticvec` for `arrayvec` and
otherwise updating some dependencies (namely `dynasm`) to compile with
stable.

This then also updates CI appropriately to not use a pinned nightly and
instead us a floating `nightly` channel so we can head off any breakage
coming up ASAP.

crates/lightbeam/src/lib.rs

@@ -1,5 +1,4 @@
 #![cfg_attr(test, feature(test))]
-#![feature(proc_macro_hygiene)]
 
 #[cfg(test)]
 extern crate test;