Add a complex benchmark

[jvoisin]

Some allocators are doing tradeoffs to perform very well on long-running complex workloads, so we should have some benchmarks to measure this. Maybe something like firefox running on a couple of webpages, or Apache?

Or maybe some lightweight javascript engine, running some javascript things.
@TheShiftedBit suggested to compile nodejs or python to use the memory allocator, and run some workload on it.