Allow to differentiate between newtypes and datatypes when using Generics

[Feuerbach]

When writing a generic instance, I would like to be able to tell whether the type under consideration has been declared using newtype or data.

For example, in SmallCheck? we generate values of certain "depth". It is natural to treat datatype constructors as increasing the depth of the constructed value, and newtype constructors as preserving the depth.

[dreixel]

I'm happy to implement this.

[jpm@…]

commit 6046b25ea35291f9fb0b30633f6f8b8d83a7a00a

Author: Jose Pedro Magalhaes <jpm@cs.ox.ac.uk>
Date:   Wed Feb 13 10:40:16 2013 +0000

    Add isNewtype to GHC.Generics (FIX #7631)

 compiler/prelude/PrelNames.lhs       |    5 +++--
 compiler/typecheck/TcGenGenerics.lhs |    7 +++++--
 2 files changed, 8 insertions(+), 4 deletions(-)

[dreixel]

Done. New test is GenNewtype.