fix: engine.Println to take strings

[Tabaie]

Currently such a test fails:

type testPrintCircuit struct {
	Var frontend.Variable
}

func (c *testPrintCircuit) Define(api frontend.API) error {
	api.Println("testing", c.Var)
	api.AssertIsEqual(c.Var, 0)
	return nil
}

func TestPrintSolve(t *testing.T) {
	circuit := testPrintCircuit{}
	assignment := testPrintCircuit{0}
	NewAssert(t).SolvingSucceeded(
		&circuit, &assignment, WithBackends(backend.GROTH16), WithCurves(ecc.BN254))
}

It is useful to have descriptive strings in print statements. This PR enables that.