Remove constraint system trait from constrained program type

[collinc97]

Overview

Currently ConstraintSystem is a parameter for ConstrainedProgram. ConstraintSystem is used in some but not all methods and is not used by ConstrainedProgram directly. This should be changed so that ConstraintSystem is only passed into methods that need it.

The current ConstrainedProgram struct:

pub struct ConstrainedProgram<F: Field + PrimeField, G: GroupType<F>, CS: ConstraintSystem<F>> {
    pub identifiers: HashMap<String, ConstrainedValue<F, G>>,
    pub _cs: PhantomData<CS>,
}

The new struct:

pub struct ConstrainedProgram<F: Field + PrimeField, G: GroupType<F>> {
    pub identifiers: HashMap<String, ConstrainedValue<F, G>>,
}

The current enforce_add_expression() method:

fn enforce_add_expression(
        &mut self,
        cs: &mut CS,
        left: ConstrainedValue<F, G>,
        right: ConstrainedValue<F, G>,
    ) -> Result<ConstrainedValue<F, G>, ExpressionError>

The new method:

fn enforce_add_expression<CS: ConstraintSystem<F>>(
        &mut self,
        cs: &mut CS,
        left: ConstrainedValue<F, G>,
        right: ConstrainedValue<F, G>,
    ) -> Result<ConstrainedValue<F, G>, ExpressionError>