Abs      : import Prelude qualified
Par/Skel : import Abs qualified

The printer already used qualified import.

Allows e.g. to use Boolean and Object as category names.

Visitors need not import the "absyn" package, as they only refer to the
syntax classes in a qualified way.

In "absyn", we refer to Object as "java.lang.Object".

Also fixes in VisitSkel:
- A layout problem: missing newlines between methods of a visitor.
- New: calls to "accept" in for-loops.

On case insensitive file systems, we cannot have public java classes
that are only distinguished by case.  Thus, report an error for clashes
modulo case, e.g. between Foo and FOO:

  Foo.  FOO ::= "foo" ;

Patched the generation of show and printing functions for abstract
syntax to refer qualified to the abstract syntax module.

Using _int instead of int_ avoids the clash with a user-defined category
"Int".

This backend does not like constructor name schema

  Foo_ . Foo ::= ...

since it uses Foo_ as tag for data type Foo.

  Foo = Tree Foo_

Use Prelude qualified in Abs module.
Use Abs module qualified in Skel module.

Needed for layout flag.

Otherwise doctest is failing. ;-)

We already test for keywords in #278, why not for "i" as well?

When translating `define`d constructors to Haskell, we need to refer to
the native AST constructors in a qualified way (after the changes
introduced in #278).

The generated Test.hs file lacks imports, regressions introduced by
work on #331 and #278.