Make phase 3 of word_list_find_intersection() optional

With the skip phase 3 optimization:

**************************************
* PERFORMANCE TESTS RESULTS          *
*                                    *
* Combined runtime: 0.790712 seconds *
* Average runtime:  0.071883 seconds *
**************************************

Without the skip phase 3 optimization:

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* PERFORMANCE TESTS RESULTS          *
*                                    *
* Combined runtime: 0.952440 seconds *
* Average runtime:  0.086585 seconds *
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