Rename baseType -> baseTypeRef

What was `baseType` and is now `baseTypeRef` only computes the prefix of of basetype, not the type arguments.
If type arguments need to be included there is `baseTypeWithArgs`. The reason is that type arguments are usually already encoded as member types. But this was a source of errors because in Scala 2, baseType includes the type arguements.

(also added test file structural.scala forgotten from last commit)