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%.

gf.cabal

@@ -57,7 +57,7 @@ flag custom-binary
   Description: Use a customised version of the binary package
   Default:     True
 
-library
+Library
   build-depends: base >= 4.2 && <5,
                  array,
                  fst,
@@ -66,11 +66,11 @@ library
                  random,
                  pretty,
                  mtl
-  hs-source-dirs: src/compiler src/runtime/haskell
+  hs-source-dirs: src/runtime/haskell
 
   if flag(custom-binary)
     hs-source-dirs: src/binary
-    other-modules:
+    exposed-modules:
       -- not really part of GF but I have changed the original binary library
       -- and we have to keep the copy for now.
       Data.Binary
@@ -81,40 +81,38 @@ library
   else
     build-depends: binary
 
+  ghc-options: -fwarn-unused-imports
   extensions: 
   exposed-modules: 
     PGF
-  other-modules:
-    PGF.CId
     PGF.Data
     PGF.Macros
+    PGF.Optimize
+    PGF.Printer
+    PGF.Signature
+    PGF.Tree
+    PGF.Utilities
+  other-modules:
+    PGF.CId
+    PGF.Binary
+    PGF.Expr
     PGF.Generate
     PGF.Linearize
-    PGF.Parse
-    PGF.SortTop
-    PGF.Expr
-    PGF.Type
-    PGF.Tree
-    PGF.Tokenizer
-    PGF.Paraphrase
-    PGF.TypeCheck
-    PGF.Binary
     PGF.Morphology
-    PGF.VisualizeTree
-    PGF.Printer
+    PGF.Paraphrase
+    PGF.Parse
     PGF.Probabilistic
+    PGF.SortTop
+    PGF.Type
+    PGF.Tokenizer
+    PGF.TypeCheck
     PGF.Forest
-    PGF.Optimize
-    PGF.Signature
-    GF.Data.TrieMap
-    GF.Data.Utilities
-    GF.Data.SortedList
-    GF.Data.ErrM
-    GF.Data.Relation
-    GF.Data.Operations
+    PGF.TrieMap
+    PGF.VisualizeTree
 
-executable gf
-  build-depends: base >= 4.2 && <5,
+Executable gf
+  build-depends: gf,
+                 base >= 4.2 && <5,
                  array,
                  containers,
                  bytestring,
@@ -131,7 +129,7 @@ executable gf
                  haskeline,
                  parallel>=3,
                  utf8-string
-  ghc-options: -threaded
+  ghc-options: -threaded -fwarn-unused-imports
   if flag(server)
     build-depends: httpd-shed, network, json, cgi
                    -- ,silently
@@ -153,21 +151,17 @@ executable gf
 
   if impl(ghc>=7.0)
     ghc-options: -rtsopts
-  hs-source-dirs: src/compiler src/runtime/haskell
+  hs-source-dirs: src/compiler
 
-  if flag(custom-binary)
-    hs-source-dirs: src/binary
-    other-modules: Data.Binary.IEEE754
-  else
+  if !flag(custom-binary)
     build-depends: binary, data-binary-ieee754
 
   extensions:
   main-is: GF.hs
   other-modules:
     GF.Data.BacktrackM
-    GF.Data.TrieMap
     GF.Data.Utilities
-    GF.Data.SortedList
+--  GF.Data.SortedList
     GF.Data.ErrM
     GF.Data.Operations
     GF.Data.Str
@@ -227,20 +221,6 @@ executable gf
     GF.Compile
     GF.Index
     GF.Quiz
-    PGF
-    PGF.CId
-    PGF.Data
-    PGF.Expr
-    PGF.Type
-    PGF.Tree
-    PGF.Macros
-    PGF.Generate
-    PGF.Linearize
-    PGF.Binary
-    PGF.Paraphrase
-    PGF.TypeCheck
-    PGF.Printer
-    PGF.Optimize
     GFC
     GFI