Efficiency tests

  		•  Test function: sin(x) + x**3 + 2*x 
    f1               :  1
StringFunction_v1: 13      (because of uncompiled eval)
StringFunction_v2:  2.3
StringFunction_v3: 22      (because of exec in __call__)
StringFunction_v4:  2.3
StringFunction_v5:  3.1    (because of loop in __call__)

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