The Arbitrary instance of the 'Instance' object is written
using the <*> syntax. Since it often uses the 'arbitrary'
generator for the instance's fields it is hard to figure
out which 'arbitrary' fills which instance field. This
patch annotates all fields with their name to make
maintenance of this code easier.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>