Make u8::is_ascii a stable const fn

`char::is_ascii` is already a stable `const fn`, so there is no reason
for `u8::is_ascii` to be unstable.

src/libcore/num/mod.rs

@@ -4300,8 +4300,9 @@ impl u8 {
     /// assert!(!non_ascii.is_ascii());
     /// ```
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.43.0")]
     #[inline]
-    pub fn is_ascii(&self) -> bool {
+    pub const fn is_ascii(&self) -> bool {
         *self & 128 == 0
     }