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/Grammar/Binary.hs

@@ -27,8 +27,8 @@ instance Binary Ident where
                 else return (identC bs)
 
 instance Binary a => Binary (MGrammar a) where
-  put (MGrammar ms) = put ms
-  get               = fmap MGrammar get
+  put = put . modules
+  get = fmap mGrammar get
 
 instance Binary a => Binary (ModInfo a) where
   put mi = do put (mtype mi,mstatus mi,flags mi,extend mi,mwith mi,opens mi,mexdeps mi,jments mi)