stats: fix UB in from_slice

[ordian]

As documented in https://doc.rust-lang.org/std/mem/union.MaybeUninit.html,

let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️

[ordian]

this copies the data unfortunately
using transmute currently not possible with const generics: rust-lang/rust#61956
but I can change it to a macro instead

[ordian]

nvm, I think it's a noop: https://godbolt.org/z/YcxonWxPa

[ordian]

The following functions are unstable, but could be used in the future:

• https://doc.rust-lang.org/std/mem/union.MaybeUninit.html#method.uninit_array
• https://doc.rust-lang.org/std/mem/union.MaybeUninit.html#method.array_assume_init

[arkpar]

Might as well get rid of unsafe here.

 let plain: [u32; 1024] = [Default::default(); 1024];
 let mut atomic: [AtomicU32; 1024] = a.map(AtomicU32::new);
 // Read into `atomic` below.

Shoudl be optimized away as well.

[ordian]

Not sure it's optimized this way though. The produced code seems different: https://godbolt.org/z/GGnfTro49
Also to sanity check, created a bench:

from_slice/Unsafe       time:   [1.0590 µs 1.0613 µs 1.0636 µs]                               
from_slice/Safe         time:   [2.1197 µs 2.1255 µs 2.1326 µs]  

[arkpar]

Ok, let's leave it as it is then.