fix clone_func not checked for being nonnull

implements cxMapIntersection() and cxMapListIntersection() - resolves #555

difference shall not check already present items in the destination - fixes #753

specify the intersection functions

add cxCollectionCompareFunc() macro

fix typo bug in cxListDifference() - resolves #745

add tests for cxListDifference() - resolves #751

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mercurial