Sieve class

A class named sieve and iterator were made to match the implementation of primesieve.org to use in ntheory functions.
Yes, you are correct, I didn't check with the master. (I  got almost the same results. 1.12 seconds and 1.23 seconds)

Since the `_extend` method is in this PR, a segmented sieve approach should work here. (0.70 seconds)
`for(unsigned i=100;i<=limit;i*=1.2)
        _extend(i);
_extend(limit)`