Make Ident abstract; imports of Data.ByteString.Char8 down from 29 to 16 modules

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.

src/compiler/GF/Speech/CFG.hs

@@ -13,7 +13,6 @@ import GF.Data.Relation
 
 import Control.Monad
 import Control.Monad.State (State, get, put, evalState)
-import qualified Data.ByteString.Char8 as BS
 import Data.Map (Map)
 import qualified Data.Map as Map
 import Data.List