Changes needed to support simple named type parameters.
Not yet implemented: named arguments.

Types#underlyingClassRef and PostTyper#normalizeTree need to be changed so they can
deal with partial named parameter lists.

After unpickling we might see an alias

    X = pre.X

where pre =:= the ThisType of the enclosing class. But it
might not be `eq` to it.

Named parameters cause some elements to be unpickled
in a different order as they are pickled. In particular
term parameter aliases and type parameter aliases seem to be swapped.

There is a before/after difference having to do with the
order in which class declarations show up.

Needs to work also if named arg refers to an abstract type,
not a parameter.

We had same fleyness in number of errors of cycle.scala
which prompted this.

Also, add an unrelated test file to pos.

Was not fixed by accident then, so we do it here now.

The current Scala spec only considers methods without default parameters
for overloading resolution (unless only a single one remains anyway after
filtering by shape). This is needlessly restrictive. But dropping this
restriction (as dotty does) can lead to ambiguity errors, which is why
run/t8197 did not compile anymore.

We fix the problem by a last try rule: If after asSpecific tests there are
still several alternatives, and only one of them is without default arguments,
pick that one.

I tried an alternative rule which would make the distinction on default params
earlier but that one fails for the overloaded tree copier functions in Trees.scala
(the method with default parameters is also the one which is more specific).

Lets one also pass named arguments to methods.