Bump base's version number to 4.12.0.0 in HEAD

Currently, base sits at version 4.11.0.0 in HEAD (which is the same version of base as shipped with GHC 8.4.1). This is making it difficult to adapt certain Hackage libraries to recent changes in base, such as moving Data.Functor.Contravariant into base (see the discussion here).

Before we can do this, here is a list of libraries that will need to have their upper version bounds on base bumped from < 4.12 to < 4.13:

• array (ready, http://git.haskell.org/packages/array.git/commit/234d0a11820c263c873e41810d7a4b9d6152dcec)
• deepseq (ready, https://github.com/haskell/deepseq/pull/41)
• directory (ready, https://github.com/haskell/directory/pull/79)
• filepath (ready, https://github.com/haskell/filepath/pull/67)
• ghc-boot (ready, in-place)
• ghc-boot-th (ready, in-place)
• ghc-compact (ready, in-place)
• ghci (ready, in-place)
• haddock (ready, https://github.com/haskell/haddock/commit/5b5058f958e6e141b9e9c12b6b79aaf218c210a7)
• haskeline (ready, https://github.com/judah/haskeline/pull/81)
• hpc (ready, http://git.haskell.org/packages/hpc.git/commit/91f520cb9725d812fa93b57bca1d8dbd72c88dd5)
• parallel (ready, https://github.com/haskell/parallel/pull/20)
• parsec (ready, https://github.com/haskell/parsec/commit/cd7a8e7abf30728f66365edb8fbbfb05fe3234ae)
• process (ready, https://github.com/haskell/process/pull/124)
• stm (ready, https://github.com/haskell/stm/pull/7)
• template-haskell (ready, in-place)
• terminfo (ready, https://github.com/judah/terminfo/pull/30)
• unix (awaiting, https://github.com/haskell/unix/pull/108)

Some of these can be changed in-place, while others will require downstream changes. I'll update this ticket as I progress.