Re-export Word from Prelude (re #9531)

The original proposal text can be found at

  http://www.haskell.org/pipermail/libraries/2014-August/023491.html

The proposal passed with a clear majority, and was additionally
confirmed by the core libraries committee.

*Compatibility Note*

Only code that imports `Data.Word` for the sole purpose of using `Word`
*and* requires to be `-Werror`-clean (due to `-fwarn-unused-imports`)
is affected by this change.

In order to write warning-free forward/backward compatible against `base`,
a variant of the following CPP-based snippet can be used:

  -- Starting with base>4.7.0 or GHC>7.8 Prelude re-exports 'Word'
  -- The following is needed, if 'Word' is the *only* entity needed from Data.Word
  #ifdef MIN_VERSION_base
  # if !MIN_VERSION_base(4,7,1)
  import Data.Word (Word)
  # endif
  -- no cabal_macros.h -- fallback to __GLASGOW_HASKELL__
  #elif __GLASGOW_HASKELL__ < 709
  import Data.Word (Word)
  #endif

This also updates the haddock submodule in order to avoid a compile warning