Node bulk allocation

This is a quick test of something I always want to try, but never did because it do not seem to be a problem when profiling scrm...

With it scrm allocates an area of memory for storing nodes at once instead of individually. Right now, this is a simple test implementation that needs more work to be merged into scrm. It is missing:

- [x] reusing of freed memory
- [x] dynamically increasing the space for nodes

But the test seems to already give a decent runtime improvement:

    time ./scrm 20 10 -r 1000 100000 -seed 20 > /dev/null                                                                                                                               
    real    0m27.233s
    user    0m27.145s
    sys     0m0.040s

    time scrm 20 10 -r 1000 100000 -seed 20 > /dev/null
    real    0m39.753s
    user    0m39.628s
    sys     0m0.008s