Most of the explicit uses of ByteStrings were eliminated by using identS,
identS = identC . BS.pack
which was found in GF.Grammar.CF and moved to GF.Infra.Ident. The function
prefixIdent :: String -> Ident -> Ident
allowed one additional import of ByteString to be eliminated. The functions
isArgIdent :: Ident -> Bool
getArgIndex :: Ident -> Maybe Int
were needed to eliminate explicit pattern matching on Ident from two modules.
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.
