Solver hints supported by test engine, signature of hints changed to big.Int

[gbotrel]

Hint functions now have the following signature:

type Function func(curveID ecc.ID, inputs []*big.Int, result *big.Int) error

and can be used like so:

func (circuit *hintCircuit) Define(curveID ecc.ID, api frontend.API) error {
	a7 := api.NewHint(mulBy7, circuit.A)
// ...
}

func mulBy7(curveID ecc.ID, inputs []*big.Int, result *big.Int) error {
	result.Mul(inputs[0], big.NewInt(7)).Mod(result, curveID.Info().Fr.Modulus())
	return nil
}

This enables to implement one hint function for all curves, and enable support of api.NewHint in the big.Int test engine.