ColorCode::new does not need to be a const fn

src/main.rs

@@ -1,5 +1,4 @@
 #![feature(lang_items)] // required for defining the panic handler
-#![feature(const_fn)] // allow declaring functions as const
 #![no_std] // don't link the Rust standard library
 #![cfg_attr(not(test), no_main)] // disable all Rust-level entry points
 #![cfg_attr(test, allow(dead_code, unused_macros))] // allow unused code in test mode