Switch MGraph to HashMap and HashSet

Hi Rob,

This is another patch on top of pull request #11 I sent (yeah, time warp). After the previous changes, rdf4h still didn't give me the performance I needed, so I switched out the Map and Set implementations. This worked pretty well for my workload, perhaps it's useful to upstream. Let me know what you think.

Note that it does not yet pass the test suite. Should be something simple, but I had to turn my attention to other things. Perhaps you see the problem immediately, otherwise I'll get to it eventually. It does not impact my use of rdf4h at the moment.

cheers, Maarten