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

const fn integer operations

A follow up to #51171
Fixes #51267

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

src/test/run-pass/const-endianess.rs

@@ -0,0 +1,32 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(const_int_ops)]
+#![feature(test)]
+
+extern crate test;
+use test::black_box as b;
+
+const BE_U32: u32 = 55u32.to_be();
+const LE_U32: u32 = 55u32.to_le();
+
+
+fn main() {
+    assert_eq!(BE_U32, b(55u32).to_be());
+    assert_eq!(LE_U32, b(55u32).to_le());
+
+    #[cfg(not(target_arch = "asmjs"))]
+    {
+        const BE_U128: u128 = 999999u128.to_be();
+        const LE_I128: i128 = -999999i128.to_le();
+        assert_eq!(BE_U128, b(999999u128).to_be());
+        assert_eq!(LE_I128, b(-999999i128).to_le());
+    }
+}