fix: Fix memory issue and repeated computation
fix: Fix memory issue and repeated computation

Sieve currently builds an array twice the size it needs to, and
filters that array every time the `#primes` method is called.

Fix the memory issue by correcting and simplifying the loop bounds.
Compute the primes in the initialize, so that we don't have to repeat
it.

Bugfry/exercism,Bugfry/exercism,Bugfry/exercises,Bugfry/exercism,Bugfry/exercises,Bugfry/exercism,Bugfry/exercises,Bugfry/exercism,Bugfry/exercises,Bugfry/exercism,Bugfry/exercises,Bugfry/exercises