Make this a struct with internally named members

[recmo]

On 2020-06-10 @pvienhage wrote in bab508e “Constraint System Contract Autogen (#709)”:

Make this a struct with internally named members
the members are (index, degree bound, expression, name)

pub enum RationalExpression {
    X,
    Constant(FieldElement),
    Trace(usize, isize),
    Polynomial(DensePolynomial, Box<RationalExpression>),
    // TODO - Make this a struct with internally named members
    // the members are (index, degree bound, expression, name)
    ClaimPolynomial(usize, usize, Box<RationalExpression>, Option<&'static str>),
    Add(Box<RationalExpression>, Box<RationalExpression>),
    Neg(Box<RationalExpression>),
    Mul(Box<RationalExpression>, Box<RationalExpression>),
    Inv(Box<RationalExpression>),

From crypto/stark/src/rational_expression.rs:23