Memory consumption

[glaslos]

Hi, first of all, thanks for this library. It was very educational reading your implementation. I run into an issue when running the benchmarks. I quickly run out of memory. Seems like b.N get's very large on my system (i7, 16GB Mem) which exhausts the memory when setting up Unset and IsSet benchmarks or running Set benchmarks. I can limit the number of element and then do %numElements in the actual benchmark for Unset and IsSet bit might skew the bechmark and doesn't solve the issue for Set. Any suggestions on how to approach this issue? Maybe limit the size of the ipmap?