Better diagnostics for failing stub count tests
New scheme for TypeTrees that refer to others

Adds a new scheme by which a TypeTree() can refer for its type to the symbol of some other type.
Applies the scheme to setter parameters, replacing the previous ad-hoc solution.
Generalize derived type tree scheme.

To get truly hygienic desugared trees, we need a derived type tree scheme that's more
flexible than just the previous two choices of info-of-symbol and typeref-of-symbol.
The new scheme based on DerivedTypeTrees allows arbitrary methods to derive the type tree's type.
Hygienic desugaring

Made desugaring hygienic. Trees that are derived from some other tree are no longer stored as simple
untyped Ident trees, but as TypeTrees that know how to derive their types from some other type.

Test cases in pos: hygiene.scala, t0054.scala and t0085.scala.

The comment in hygiene.scala points to the difficulties we are facing. In particular, we need type
trees that can rebind some references of a source type to local occurrences with the same name. t0054.scala is similar to hygiene.scala. t0085.scala is trickier, but also related. Essentially the problem there is that we have a class
that inherits its outer class. In this case it is important to resolve an identifier in the right context. The identifier added to the copy method of a case class must be resolved outside the class (just like the same
identifier in the constructor of that case class).
Minor improvements to tests (remove old comments)