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/Rename.hs

@@ -62,7 +62,7 @@ renameSourceJudgement g m (i,t) = do
 renameModule :: [SourceModule] -> SourceModule -> Check SourceModule
 renameModule ms (name,mo) = checkIn (text "renaming module" <+> ppIdent name) $ do
   let js1 = jments mo
-  status <- buildStatus (MGrammar ms) name mo
+  status <- buildStatus (mGrammar ms) name mo
   js2    <- checkMap (renameInfo status name) js1
   return (name, mo {opens = map forceQualif (opens mo), jments = js2})
 
@@ -128,7 +128,7 @@ tree2status o = case o of
 
 buildStatus :: SourceGrammar -> Ident -> SourceModInfo -> Check Status
 buildStatus gr c mo = let mo' = self2status c mo in do
-    let gr1 = MGrammar ((c,mo) : modules gr)
+    let gr1 = mGrammar ((c,mo) : modules gr)
         ops = [OSimple e | e <- allExtends gr1 c] ++ opens mo
     mods <- checkErr $ mapM (lookupModule gr1 . openedModule) ops
     let sts = map modInfo2status $ zip ops mods