Allow mappings to case classes with 22 instead of just 20.

I was able to figure out the problem that was preventing builders from supporting 22 fields. It turns out it was the omission of some type params that were supposed to be there. I'm honestly not entirely sure how it compiled before, because those type parameters should definitely be there. 

I also changed the output to use string interpolation for clarity, which maybe I shouldn't have done at this moment, since it made the diff a little harder to read. But the only part I changed was  

this: `new Builder${arity+1}(combi(m1, m2), m3)`

to this `new Builder${arity+1}${typeParamsInc}(combi(m1, m2), m3)` 

On my SBT, the whole thing compiles in just a few seconds, and there are no memory errors. I used a heap size of 1G. Have not tried other sizes.