Dropping support for NHC98 and old GHC from containers

[David Feuer]

The containers package currently includes CPP and Cabal directives
intended to support NHC98 and versions of GHC before 7.0. However, our
TravisCI script tests only GHC versions 7.0 and above. It is therefore
likely that the compatibility code is bit-rotted to some extent. Wren
Romano and I are strongly inclined to immediately remove all code
intended to support NHC98 or pre-7.0 GHC. If anyone uses any such
implementations with the current version of containers, please speak
now or forever hold your peace.

Thanks,
David Feuer