refactor linalg tests

in this MR, i define the following two functions in linalg::tests

• vec_to_elements<T: Field>(elements: Vec<u128>) -> Vec<T>
• mat_to_elements<T: Field>(mat: Vec<Vec<u128>>) -> Vec<Vec<T>>

the idea is to help see what the matrices and vectors are at a glance, without too much processing. the end result is that all Fr::from(<some number>) are gone

example

• a full matrix

Matrix::from_vec_vec(vec![
    vec![Fr::from(2), Fr::zero(), Fr::zero()],
    vec![Fr::zero(), Fr::from(3), Fr::zero()],
    vec![Fr::zero(), Fr::zero(), Fr::from(4)],
    vec![Fr::from(2), Fr::from(3), Fr::from(4)],
])

becomes

Matrix::<Fr>::from_vec_vec(mat_to_elements(vec![
    vec![2, 0, 0],
    vec![0, 3, 0],
    vec![0, 0, 4],
    vec![2, 3, 4],
]))

which is hopefully easier to read and understand what the matrix is.

• a diagonal one

Matrix::<Fr>::from_diagonal(vec![Fr::from(2), Fr::from(3), Fr::from(4)])

becomes

Matrix::<Fr>::from_diagonal(vec_to_elements(vec![2, 3, 4]))