AdjacencyMatrixGraph: store rows in vector<uint64_t> instead of vector<bool>

Incurs a weird 20% performance hit for the existing algorithms, but
enables efficient whole-row operations such as OR.
Switch from 'count' output to 'matrix' for benchmarking

Now that we're getting to algorithms that cannot be implemented for
'count', we really should switch to the output format prescribed in
the assignment.
Implement RecMerge: simple recursive reachability merging

flatline-awesome on dense, surprisingly-not-bad on sparse graphs