Expose GF.Grammar.Canonical + some refactoring

to make it available in other tools by depending on the gf package and
importing it

gf.cabal

@@ -151,6 +151,7 @@ Library
     GF.Support
     GF.Text.Pretty
     GF.Text.Lexing
+    GF.Grammar.Canonical
 
   other-modules:
     GF.Main GF.Compiler GF.Interactive
@@ -190,7 +191,6 @@ Library
     GF.Haskell
     GF.Compile.ConcreteToHaskell
     GF.Compile.GrammarToCanonical
-    GF.Grammar.Canonical
     GF.Grammar.CanonicalJSON
     GF.Compile.PGFtoJS
     GF.Compile.PGFtoProlog