Rollup merge of rust-lang#51299 - faern:const-int-ops, r=oli-obk

const fn integer operations

A follow up to rust-lang#51171
Fixes rust-lang#51267

Makes a lot of the integer methods (`swap_bytes`, `count_ones` etc) `const fn`s. See rust-lang#51267 for a discussion about why this is wanted and the solution used.

Author: Mark-Simulacrum

src/libcore/lib.rs

@@ -81,6 +81,7 @@
 #![feature(cfg_target_has_atomic)]
 #![feature(concat_idents)]
 #![feature(const_fn)]
+#![feature(const_int_ops)]
 #![feature(core_float)]
 #![feature(custom_attribute)]
 #![feature(doc_cfg)]