Factor: 7: Implement my own sieve

Although the sieve in factor is quite good, it is a good exercise to
implement my own to solve these problems.
Factor: Cleanup on get.py

Put the USING: declarations on individual lines to make it easier to
edit.  Remove the 'prettyprint' since we're not using that.
Factor: 9-11
Factor: 12-17
Factor: 18-20