Replace uses of MaybeUninit::uninit_array() with inline const blocks.

[kpreid]

[This PR originally contained the changes in #125995 too. See edit history for the original PR description.]

The documentation of MaybeUninit::uninit_array() says:

Note: in a future Rust version this method may become unnecessary when Rust allows inline const expressions. The example below could then use let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];.

The PR adding it also said: #65580 (comment)

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];

That time has come to pass — inline const expressions are stable — so MaybeUninit::uninit_array() is now unnecessary. The only remaining question is whether it is an important enough convenience to keep it around.

I believe it is net good to remove this function, on the principle that it is better to compose two orthogonal features (MaybeUninit and array construction) than to have a specific function for the specific combination, now that that is possible.