Switch back to core::array::from_fn once rustc fix has been around long enough

[HadrienG2]

In this crate, I had to reimplement core::array::from_fn because it had bad codegen on then-current rustc releases. However, a fix has landed in nightly and should be making its way to 1.69 eventually: rust-lang/rust#108765 .

This raises the question of when it is appropriate to bump the minimal rustc requirement for good codegen, in a crate where runtime performance is the whole point. For the time being, my custom array::from_fn does not harm much and can stay around, but in a couple of years, once rustc 1.67 and 1.68 are old news, I should consider dropping it.