Implement histogramming

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Right now, MULTISHOT or MULTISHOT-MEASURE gives bitstrings back. Often it's only necessary to look at a bunch of (uncorrelated) statistics. The QVM maybe should give a way of just giving histograms of |0>/|1> measurements back.

Now, of course this can be written in Quil directly with arithmetic instructions, but understandably, not all hardware easily supports this. If we implemented this as a special API case, we could restrict it to work on single BITs of memory.