Good news from the garbage collection front

It seems that we can do better! Armin fixed a bug in our generational garbage collector, which caused variable sized objects (e.g. arrays) to be allocated outside of the nursery. This resulted in 50% speedup on synthetic benchmarks and about 10-20% on real world ones. Doing some preliminary measures, it seems that we spend roughly 10% of the time in garbage collection, which is good (and there is still some room for improvements!)