fix inconsistent destructor requirements for list and map classes

fix rehash not valid for non-hash-maps

add empty map implementation - fixes #259

fix cxListCompare if-condition

add empty list implementation - fixes #258

improve cx_linked_list_sort() - fixes #257

fix cx_linked_list_sort() not working for empty lists

explicitly set cmake policy CMP0077

add cx_strdup_m() and cx_strdup_ma()

make list find return a negative value when elem not found

(0) -300 -100 -10 +10 +100 tip

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