Parser: Improve style

[magnus-madsen]

General:

• Avoid or-patterns
• Re-enable ignored tests

Weeder2:

• Rework getDuplicates implementation.
• Move string constants to fields at the top (e.g. "Debug.stringify")

[mlutze]

May I request:

• Avoid name shadowing
• Use xyz0, xyzVal, xyz format, which is the standard for the front-end

def foo(abc: ABC): Validation[ABC] = abc match {
   case ABC(xyz0, qrs0) =>
     val xyzVal = visitXYZ(xyz0) // returns validation
     val qrs = visitQRS(qrs0) // returns non-validation
     mapN(xyzVal) {
       case xyz => ABC(xyz, qrs)
     }
}