Switch to GHC 7.8

GHC 7.8 removed all singleton functionality from GHC.TypeLits
(base-4.6 is not supported)

-fcontext-stack=200 is used to avoid Partner module compilation
error (see also https://ghc.haskell.org/trac/ghc/ticket/5395 and
ghc/ghc@b6b5c4179b3363f2ceafc55e64b545316c11dc26)