Provide Typeable1..7 as type synonyms (see #8813)

(cherry picked from commit a5383f7400d05a758094578da28d958fb694b726)
Tweak Haddock markup in Data.Typeable

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
(cherry picked from commit c7e0ea387bc9364bb9af33aec0f710db19765963)