Fixes missing generic_const_exprs feature label

This was causing an ICE before, now everything runs again! Submitted the bug
to the Rust Github issues tracker here:
rust-lang/rust#88882

Author: gkanwar

src/bin/test_simple.rs

@@ -1,3 +1,5 @@
+#![feature(generic_const_exprs)]
+
 use lqft::{c64, Action, Boundaries, ColorMat, ColorMatrix, Lattice4ColorMat, PureGaugeAction};
 use ndarray::Array4;