GF.Infra.Modules: keep the modules of a grammar in a finite map instead of a list

This speeds up the compilation of PhrasebookFin.pgf by 12%, mosly by speeding
up calls to lookupModule in calls from lookupParamValues, in calls
from allParamValues.

The invariant "modules are stored in dependency order" is no longer respected!
But the type MGrammar is now abstract, making it easier to maintain this or
other invariants in the future.

src/compiler/GF/Compile.hs

@@ -69,8 +69,8 @@ batchCompile opts files = do
 
 -- to compile a set of modules, e.g. an old GF or a .cf file
 compileSourceGrammar :: Options -> SourceGrammar -> IOE SourceGrammar
-compileSourceGrammar opts gr@(MGrammar ms) = do
-  (_,gr',_) <- foldM compOne (0,emptySourceGrammar,Map.empty) ms
+compileSourceGrammar opts gr = do
+  (_,gr',_) <- foldM compOne (0,emptySourceGrammar,Map.empty) (modules gr)
   return gr'
  where
   compOne env mo = do
@@ -215,19 +215,19 @@ generateModuleCode opts file minfo = do
 
 -- auxiliaries
 
-reverseModules (MGrammar ms) = MGrammar $ reverse ms
+--reverseModules (MGrammar ms) = MGrammar $ reverse ms
 
 emptyCompileEnv :: CompileEnv
 emptyCompileEnv = (0,emptyMGrammar,Map.empty)
 
-extendCompileEnvInt (_,MGrammar ss,menv) k mfile sm = do
+extendCompileEnvInt (_,gr,menv) k mfile sm = do
   let (mod,imps) = importsOfModule sm
   menv2 <- case mfile of
     Just file -> do
       t <- ioeIO $ getModificationTime file
       return $ Map.insert mod (t,imps) menv
     _ -> return menv
-  return (k,MGrammar (sm:ss),menv2) --- reverse later
+  return (k,mGrammar (sm:modules gr),menv2) --- reverse later
 
 extendCompileEnv e@(k,_,_) file sm = extendCompileEnvInt e k (Just file) sm