miri; fix unsafe ub

[maksimryndin]

• adding miri to ci to detect undefined behaviour (UB) as the codebase has quite a lot of unsafe code
• refactoring some tests along the way (to deduplicate the code)
• fixing UB behaviour (memory alignment violation) by removing the method into_cpu_vec as it violates the invariant of the same element alignment. We use a slice instead. We are forced to allocate in new_canonical_ordered but it seems not to be on the critical path so it should be no issue.

The alignment invariant violation can be reproduced with the program (cargo +nightly miri run)

#![feature(portable_simd)]
#![feature(vec_into_raw_parts)]

use std::mem;
use std::simd::Simd;

fn main() {
    let array: [u32; 16] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15];

    let packed: Simd<u32, 16> = array.into();

    println!("PackedBaseField alignment: {} bytes", std::mem::align_of::<Simd<u32, 16>>());
    println!("BaseField alignment: {} bytes", std::mem::align_of::<u32>());

    let mut col = vec![packed];
    let capacity = col.capacity() * 16;
    let length = 8;
    let ptr = col.as_mut_ptr() as *mut u32; // aka BaseField

    let res = unsafe { Vec::from_raw_parts(ptr, length, capacity) };
    mem::forget(col);

    println!("{:?}", res)
}

[reviewable-StarkWare]

This change is