update src/utils.rs methods to work on F instead of on G

[arnaucube]

SparseMatrix does not need G but just G::Scalar, thus maybe makes more sense to abstract it around F instead of aroung G.

This also applies to the rest of methods of SparseMatrix and to the standalone methods sparse_vec_to_mle and matrix_vector_product_sparse.
In the same way that other methods of src/utils.rs are with generic F, such as dense_vec_to_mle, matrix_vector_product, etc.

This will affect also the tests, where instead of for example
test_matrix_vector_product_sparse_with<G: Group>()
it could be
test_matrix_vector_product_sparse_with<F: PrimeField>()