New test case

A simplified test case which adopts several improvements from Miles Sabin's branch.
Compared to typeclass-derivation2c.scala:

 - The alternatives and numberOfCases methods are gone
 - All compiler-generated info is in the companion objects;
   no additional classes are generated.
 - The derivation code largely uses the type traversals of
   the previous schemes instead of simple counting, as was
   the case in typeclass-derivation2c.scala.
 - Generic has been renamed to Mirror, following a suggestion
   by Miles (but now all mirrors are static)

The key insight is that we do not need a mapping from the sum type
to its alternatives except knowing what the alternative types are.
Once we have an alternative type, we can use an implicit match
to retrieve the mirror for that alternative.

Author: odersky

tests/run/typeclass-derivation2d.check

@@ -0,0 +1,10 @@
+ListBuffer(0, 11, 0, 22, 0, 33, 1)
+Cons(11,Cons(22,Cons(33,Nil)))
+ListBuffer(0, 0, 11, 0, 22, 0, 33, 1, 0, 0, 11, 0, 22, 1, 1)
+Cons(Cons(11,Cons(22,Cons(33,Nil))),Cons(Cons(11,Cons(22,Nil)),Nil))
+ListBuffer(1, 2)
+Pair(1,2)
+Cons(hd = 11, tl = Cons(hd = 22, tl = Cons(hd = 33, tl = Nil())))
+Cons(hd = Cons(hd = 11, tl = Cons(hd = 22, tl = Cons(hd = 33, tl = Nil()))), tl = Cons(hd = Cons(hd = 11, tl = Cons(hd = 22, tl = Nil())), tl = Nil()))
+Cons(hd = Left(x = 1), tl = Cons(hd = Right(x = Pair(x = 2, y = 3)), tl = Nil()))
+Cons(hd = Left(x = 1), tl = Cons(hd = Right(x = Pair(x = 2, y = 3)), tl = Nil()))