Simplify custom Shape definitions
Add HList and Nat implementations with Shapes for the HList.

- Simplify MapperTest.testCustomShape. No need to introduce
  another HList here, now that we have a real one in Slick.

Tests in HListTest, NatTest and MapperTest.testHList.
Merge pull request #208 from slick/tmp/shape-improvements

HLists for Slick