add arb instances for string & alpha num string, migrate to gulp, add sized generators

Review by @paf31 or @garyb 
`quickCheckPure`` allows specification of the size, not just the starting seed. Maybe what I really want is a `quickCheck'`.

I'd be in favor of deleting the non-primed versions and using those names for the primed versions if you think it's OK to break backward compatibility a bit...