closes #7 (Check if bsearch is better than include?)

yes, it is better.

With include?:
problem 1 result: 233168 (0.0 sec)
problem 2 result: 4613732 (0.0 sec)
problem 3 result: 6857 (66.43 sec)
problem 4 result: 906609 (0.49 sec)
problem 5 result: 232792560 (0.0 sec)
problem 6 result: 25164150 (0.0 sec)
problem 7 result: 104743 (0.07 sec)
problem 8 result: 40824 (0.0 sec)
problem 9 result: 31875000 (0.01 sec)
problem 10 result: 142913828922 (0.07 sec)
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All finished in 67.07 sec.

with binary_search:
problem 1 result: 233168 (0.0 sec)
problem 2 result: 4613732 (0.0 sec)
problem 3 result: 6857 (0.15 sec)
problem 4 result: 906609 (0.54 sec)
problem 5 result: 232792560 (0.0 sec)
problem 6 result: 25164150 (0.0 sec)
problem 7 result: 104743 (0.06 sec)
problem 8 result: 40824 (0.0 sec)
problem 9 result: 31875000 (0.01 sec)
problem 10 result: 142913828922 (0.08 sec)
-----
All finished in 0.85 sec.