Annotate each reverse_bits with #[must_use]

Because the name sounds like an in-place mutation like
`[T]::reverse(&mut self)`, it may be confused for one.

src/libcore/num/mod.rs

@@ -472,6 +472,7 @@ assert_eq!(m, ", $reversed, ");
             #[unstable(feature = "reverse_bits", issue = "48763")]
             #[rustc_const_unstable(feature = "const_int_conversion")]
             #[inline]
+            #[must_use]
             pub const fn reverse_bits(self) -> Self {
                 (self as $UnsignedT).reverse_bits() as Self
             }
@@ -2521,6 +2522,7 @@ assert_eq!(m, ", $reversed, ");
 ```"),
             #[unstable(feature = "reverse_bits", issue = "48763")]
             #[inline]
+            #[must_use]
             pub const fn reverse_bits(self) -> Self {
                 intrinsics::bitreverse(self as $ActualT) as Self
             }

src/libcore/num/wrapping.rs

@@ -524,6 +524,7 @@ assert_eq!(n.trailing_zeros(), 3);
             /// ```
             #[unstable(feature = "reverse_bits", issue = "48763")]
             #[inline]
+            #[must_use]
             pub const fn reverse_bits(self) -> Self {
                 Wrapping(self.0.reverse_bits())
             }