WIP: Optimize primes_less_than

Refactor: Rename array_of_primes_less_than to primes_less_than.
Decouple method name from implementation.

Refactor: Move problem-specific method to primes library.
Initial benchmark for primes_less_than

Refactor(prob10): remove crufty code
Bugfix: prob3 wasn't updated for name refactor
Update Prob3 solution to use prime_factorization_of

Didn't create benchmark because ported implementation was taking over 10
minutes to run.

It's likely the 'divisors_of' rebuild re-implemented its logic in such a
way that broke this solution (for practical runtimes).
Add ruby performance benchmarks

Related to Prob10
Add generic benchmark template
Optimize primes_less_than
Clean up primes.rb