Cleanups in type printing.

More consistency as to how to understand aliases,
singletons, specialized symbols, subclasses. Fewer
weird special casings, like normalizing tuples
and functions during type printing, but nothing else.
I avoid "normalize" entirely now, and do not make
special cases for dealiasing, which is already well
handled when printing error messages.

Look at the change to test/files/neg/t2641.check to get
a sense of why we should resist calling normalize during
the early days of a compilation run.

Anonymous and refinement classes can be printed far more
usefully by revealing their parents, and that too is here.

Hardened toString against undesirable side effects.
Make toString final to discourage any further rogue
toString overriders. Make safeToString a little safer.
Make -Ytyper-debug output readable.
Crashes now unroll more like this:

// SI-7406
class Arne[@specialized(Int) T](x: T) { lazy val y = x }

% scalac a.scala
error: assertion failed: Arne$mcI$sp.this.$asInstanceOf[Int.type]().unbox

  at scala.tools.nsc.ast.TreeGen.mkCast(TreeGen.scala:146)
  at scala.tools.nsc.Global$gen$.mkAttributedCast(Global.scala:106)
  at scala.tools.nsc.transform.Erasure$Eraser.cast(Erasure.scala:636)
  at scala.tools.nsc.transform.Erasure$Eraser.scala$tools$nsc$transform$Erasure$Eraser$$adaptToType(Erasure.scala:665)
  at scala.tools.nsc.transform.Erasure$Eraser.adapt(Erasure.scala:765)
  at scala.tools.nsc.typechecker.Typers$Typer.runTyper$1(Typers.scala:5345)
  at scala.tools.nsc.typechecker.Typers$Typer.scala$tools$nsc$typechecker$Typers$Typer$$typedInternal(Typers.scala:5358)
  at scala.tools.nsc.typechecker.Typers$Typer.body$3(Typers.scala:5305)
  at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:5310)
  at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$60.apply(Typers.scala:3021)

     while compiling: a.scala
        during phase: mixin
     library version: version 2.11.0-20130608-020907-869ebd8bf2
    compiler version: version 2.11.0-20130608-041104-feb660ccd7

one error found