Subtraction in linear combination

[mmaker]

Currently we can set subtraction in a LinearCombination in the following way:

    let mut linear_relation = LinearRelation::<G>::new();
    let x_var = linear_relation.allocate_scalar();
    let B_var = linear_relation.allocate_element();
    let X_var = linear_relation.allocate_eq((x_var + (-G::Scalar::from_u128(1u128))) * B_var);
    linear_relation.set_element(B_var, B);
    linear_relation.set_element(X_var, X);

as in https://github.com/mmaker/sigma-rs/blob/main/src/tests/test_relations.rs#L280-L286.

It would be ideal to write more simply.

(x_var - G::Scalar::from_u128(1u128)) * B_var

and even more simply

(x_var - 1) * B_var

This issue is to keep track of this.