Use inline const blocks to create arrays of MaybeUninit; remove uninit_array().

[kpreid]

This PR contains 3 commits, arranged in order of increasing contentiousness. All of them are enabled by the fact that inline_const is now stable.

1. Use inline const instead of unsafe to construct arrays in MaybeUninit examples.

   This should be straightforwardly good, since it eliminates now-unnecessary unsafe code in a user-facing example.

2. Use inline const instead of unsafe to implement MaybeUninit::uninit_array().

   This is arguably giving the compiler more work to do, in exchange for eliminating just one single internal unsafe block.

3. Delete MaybeUninit::uninit_array() and replace its uses with array expressions.

   This idea was even mentioned as an alternative when uninit_array() was added: Add MaybeUninit methods uninit_array, slice_get_ref, slice_get_mut #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];

   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 this reason to want uninit_array() — unless the repeat syntax is too annoying, or the code generation is worse (on some simple test cases, it doesn't seem to be).

   I believe it is 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.

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[ChayimFriedman2]

I still prefer MaybeUninit::uninit_array() to [const { MaybeUninit::uninit() }; N], it's shorter and more readable IMHO.

[workingjubilee]

The examples and the compiler code should probably still prefer the stable option.

Technically the compared-against example was [MaybeUninit::uninit(); N], which implies that the array initializer is a const context.