Use TemplateHaskell instead of hand-coded instances

This patch replaces the current hard-coded JSON instances (all alike,
just manual conversion to/from string) with auto-generated code based
on Template Haskell
(http://www.haskell.org/haskellwiki/Template_Haskell

).

The reduction in code line is not big, as the helper module is well
documented and thus overall we gain about 70 code lines; however, if
we ignore comments we're in good shape, and any future addition of
such data types will be much simpler and less error-prone.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>