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/lib.rs

@@ -140,7 +140,6 @@
 #![feature(const_likely)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_assume_init)]
-#![feature(const_maybe_uninit_uninit_array)]
 #![feature(const_nonnull_new)]
 #![feature(const_num_midpoint)]
 #![feature(const_option)]
@@ -177,7 +176,6 @@
 #![feature(is_ascii_octdigit)]
 #![feature(isqrt)]
 #![feature(link_cfg)]
-#![feature(maybe_uninit_uninit_array)]
 #![feature(offset_of_enum)]
 #![feature(offset_of_nested)]
 #![feature(panic_internals)]