GH-2169: Enhanced GraphMem Cloning:
- added new interface Copyable<>
  - implemented Copyable#copy in GraphMem2 all three descendants (Legacy, Fast and Roaring)
  - extended TripleStore to implement Copyable<> and implemented it in all three descendants (Legacy, Fast and Roaring)
- added copy constructors to mem2/collections/*, mem2/store/*/*Bunch, mem2/store/*/*HashMap and mem2/store/*/*Set
- added unit tests to keep the test coverage for mem2/** at 100%
- added benchmark for GraphMem2#copy
- implemented new method G#copy which uses Copyable#copy if the graph implements it, otherwise G#copyGraphSrcToDst is used as fallback.
  - added a unit test for G#copy
40 files changed