Relax the restriction on using abstract newtypes in FFI declarations.
Given the high impact of this change, we decided to back off and make abstract newtypes give a warning for one release, before we make it an error in 7.6.1. Codec/Compression/Zlib/Stream.hsc:884:1: Warning: newtype `CInt' is used in an FFI declaration, but its constructor is not in scope. This will become an error in GHC 7.6.1. When checking declaration: foreign import ccall unsafe "static zlib.h deflate" c_deflate :: StreamState -> CInt -> IO CInt