Clean up warnings and restore GHC 7.6 compatibility

This commit

 - removes redundant imports
 - removes obsolete `{-# OPTIONS_GHC -#include "HsBase.h" #-}`
 - adds a forgotten guard to one `isTrue#` occurence
 - adds a few explicit `_ <-` binds to avoid unused-binds warning
 - relax `base` build-dep version constraint to include GHC 7.6
 - remove warning-disabling `OPTIONS_GHC` from modules
 - adds `ghc-options: -Wall` to `array.cabal` file as the code base
   now warning-free on GHC 7.6 and GHC HEAD

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>