ORM to Slick

review by @szeiger 

Please already review this. I keep polishing it as we go a long.
This shouldn't happen. MappedTo is a universal trait. And we have a test case where it works: https://github.com/slick/slick/blob/master/slick-testkit/src/main/scala/com/typesafe/slick/testkit/tests/RelationalMapperTest.scala#L138