Add #[must_use] to Array::map

The output of Array::map is intended to be an array of the same size, and does not modify the
original in place nor is it intended for side-effects. Thus, under normal circumstances it should be consumed.

See [discussion](https://internals.rust-lang.org/t/array-map-annotate-with-must-use/22813/26).

Attaching to tracking issue #75243

Author: JulianKnodt

library/core/src/array/mod.rs

@@ -531,6 +531,7 @@ impl<T, const N: usize> [T; N] {
     /// let y = x.map(|v| v.len());
     /// assert_eq!(y, [6, 9, 3, 3]);
     /// ```
+    #[must_use]
     #[stable(feature = "array_map", since = "1.55.0")]
     pub fn map<F, U>(self, f: F) -> [U; N]
     where