Replace MaybeUninit::uninit_array() with array repeat expression.

This is possible now that inline const blocks are stable; the idea was
even mentioned as an alternative when `uninit_array()` was added:
<https://github.com/rust-lang/rust/pull/65580#issuecomment-544200681>

> if it’s stabilized soon enough maybe it’s not worth having a
> standard library method that will be replaceable with
> `let buffer = [MaybeUninit::<T>::uninit(); $N];`

Const array repetition and inline const blocks are now stable (in the
next release), so that circumstance has come to pass, and we no longer
have reason to want `uninit_array()` other than convenience. Therefore,
let’s evaluate the inconvenience by not using `uninit_array()` in
the standard library, before potentially deleting it entirely.

library/core/src/array/mod.rs

@@ -127,7 +127,7 @@ where
     R: Try,
     R::Residual: Residual<[R::Output; N]>,
 {
-    let mut array = MaybeUninit::uninit_array::<N>();
+    let mut array = [const { MaybeUninit::uninit() }; N];
     match try_from_fn_erased(&mut array, cb) {
         ControlFlow::Break(r) => FromResidual::from_residual(r),
         ControlFlow::Continue(()) => {
@@ -918,7 +918,7 @@ impl<T> Drop for Guard<'_, T> {
 pub(crate) fn iter_next_chunk<T, const N: usize>(
     iter: &mut impl Iterator<Item = T>,
 ) -> Result<[T; N], IntoIter<T, N>> {
-    let mut array = MaybeUninit::uninit_array::<N>();
+    let mut array = [const { MaybeUninit::uninit() }; N];
     let r = iter_next_chunk_erased(&mut array, iter);
     match r {
         Ok(()) => {