Text.Pandoc.JSON: Use To/FromJSON instances from pandoc-types.

* These use GHC generics rather than syb, and are faster.
* toJsonFilter is now a deprecated synonym of toJSONFilter from
  Text.Pandoc.JSON.
* The deprecated jsonFilter function has been removed.
pandoc.cabal: Removed support for ghc version < 7.2.

There is no point to supporting them, since pandoc-types
requires at least ghc 7.2 for GHC generics.
Text.Pandoc: Don't reexport ToJSONFilter.

It's better just to import this from Text.Pandoc.JSON.
That way, compiled filters will be smaller in size.