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/SubExOpt.hs

@@ -61,7 +61,7 @@ unsubexpModule sm@(i,mo)
       Q (m,c) | isOperIdent c -> --- name convention of subexp opers
         errVal t $ liftM unparTerm $ lookupResDef gr (m,c)
       _ -> C.composSafeOp unparTerm t
-    gr = M.MGrammar [sm] 
+    gr = M.mGrammar [sm]
     rebuild = buildTree . concat
 
 -- implementation