Eliminate mutual dependencies between the GF compiler and the PGF library

+ References to modules under src/compiler have been eliminated from the PGF
  library (under src/runtime/haskell). Only two functions had to be moved (from
  GF.Data.Utilities to PGF.Utilities) to make this possible, other apparent
  dependencies turned out to be vacuous.

+ In gf.cabal, the GF executable no longer directly depends on the PGF library
  source directory, but only on the exposed library modules. This means that
  there is less duplication in gf.cabal and that the 30 modules in the
  PGF library will no longer be compiled twice while building GF.

  To make this possible, additional PGF library modules have been exposed, even
  though they should probably be considered for internal use only. They could
  be collected in a PGF.Internal module, or marked as "unstable", to make
  this explicit.

+ Also, by using the -fwarn-unused-imports flag, ~220 redundant imports were
  found and removed, reducing the total number of imports by ~15%.

src/runtime/haskell/PGF/Utilities.hs

@@ -0,0 +1,20 @@
+-- | Basic utilities
+module PGF.Utilities where
+import Data.Set(empty,member,insert)
+
+
+-- | Like 'nub', but O(n log n) instead of O(n^2), since it uses a set to lookup previous things.
+--   The result list is stable (the elements are returned in the order they occur), and lazy.
+--   Requires that the list elements can be compared by Ord.
+--   Code ruthlessly taken from http://hpaste.org/54411
+nub' :: Ord a => [a] -> [a]
+nub' = loop empty
+    where loop _    []            = []
+          loop seen (x : xs)
+              | member x seen = loop seen xs
+              | otherwise         = x : loop (insert x seen) xs
+
+
+-- | Replace all occurences of an element by another element.
+replace :: Eq a => a -> a -> [a] -> [a]
+replace x y = map (\z -> if z == x then y else z)