Stabilize `str::from_utf8_unchecked` as `const` This stabilizes `unsafe fn str::from_utf8_unchecked` as `const` pending FCP on #75196. By the time FCP finishes, the beta will have already been cut, so I've set 1.55 as the stable-since version. (should also be +relnotes but I don't have the permission to do that) r? `@m-ou-se` Closes #75196