Add position information to types and data constructors #2216

This would allow psc-ide to jump to the definition site of data constructors, aswell as psc-docs to extract docstrings for them.

We could also improve Errors around mistakes when declaring data constructors. For example:

data List a
  = Nil
  | Cons a List

Currently points at the data with the error message:

  Could not match kind

    * -> *

  with kind

    *

in type constructor List

I think it would be a lot better to point the user at the data constructor Cons instead of the type constructor List, because that suggests he should be changing the LHS of his definition.