Will panic due to use of mem::uninitialized in fixed_array.rs

Hello! I'm coming to this repo from https://github.com/rust-lang/rust/pull/99389 where this crate panicked under a stricter check for mem::uninitialized().

The best way to handle this would be to use https://doc.rust-lang.org/std/mem/union.MaybeUninit.html and initialize a MaybeUninit of the array, then, at the end, call `assume_init` on it. I'd be happy to help with this.

Or, for more up to date version of rust, see https://doc.rust-lang.org/std/array/fn.from_fn.html which is becoming stable soon.

Start by reading fixed_array.rs and the linked Rust documentation for MaybeUninit and array::from_fn. Compare the current mem::uninitialized path with the proposed initialization approach. Done means fixed_array.rs no longer relies on mem::uninitialized and the panic described in the issue is addressed.

Written by the indexing model from the issue text.