QDate looked up lengths of months in an array. Change it to use some
simple arithmetic instead. Benchmark result unchanged:
RESULT : tst_QDate::monthLengths():
0.33 msecs per iteration (total: 87, iterations: 256)
Change-Id: I1ab0fa3b97e6716598e50d643a498e0b01c04a96
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>