Remove stable #![feature] and most x86_64 static mut.

src/main.rs

@@ -42,19 +42,16 @@
 // Ensure that all must_use results are used
 #![deny(unused_must_use)]
 #![feature(allocator_api)]
-#![feature(asm_const)]
 #![feature(core_intrinsics)]
 #![allow(internal_features)]
 #![feature(int_roundings)]
 #![feature(iter_next_chunk)]
 #![feature(let_chains)]
 #![feature(naked_functions)]
-#![feature(new_uninit)]
 #![feature(sync_unsafe_cell)]
 #![feature(variant_count)]
 #![cfg_attr(not(test), no_std)]
 #![cfg_attr(not(test), no_main)]
-#![feature(option_get_or_insert_default)]
 #![feature(array_chunks)]
 #![feature(if_let_guard)]
 #![feature(iterator_try_collect)]