Clean up new bloom filter implementation

[dbaggerman]

Cleaned up to use math/rand to distribute the numbers rather than handrolled maths.

It seems like the better number distribution leads to a slightly more efficient bloom filter (as I suspected). It's not a huge improvement, but 2-3% faster is still a win.

Benchmark #1: ./scc-master ../../torvalds/linux
  Time (mean ± σ):     699.7 ms ±   7.6 ms    [User: 9.277 s, System: 0.908 s]
  Range (min … max):   683.8 ms … 716.0 ms    50 runs

Benchmark #2: ./scc-bloom2 ../../torvalds/linux
  Time (mean ± σ):     678.1 ms ±   9.5 ms    [User: 8.846 s, System: 0.929 s]
  Range (min … max):   653.0 ms … 698.4 ms    50 runs

Summary
  './scc-bloom2 ../../torvalds/linux' ran
    1.03 ± 0.02 times faster than './scc-master ../../torvalds/linux'