Represent identifiers as UTF-8-encoded ByteStrings

This was a fairly simple change thanks to previous work on making the Ident
type abstract and the fact that PGF.CId already uses UTF-8-encoded
ByteStrings.

One potential pitfall is that Data.ByteString.UTF8 uses the same type for
ByteStrings as Data.ByteString. I renamed ident2bs to ident2utf8 and
bsCId to utf8CId, to make it clearer that they work with UTF-8-encoded
ByteStrings.

Since both the compiler input and identifiers are now UTF-8-encoded
ByteStrings, the lexer now creates identifiers without copying any characters.
**END OF DESCRIPTION***

Place the long patch description above the ***END OF DESCRIPTION*** marker.
The first line of this file will be the patch name.


This patch contains the following changes:

M ./src/compiler/GF/Compile/CheckGrammar.hs -3 +3
M ./src/compiler/GF/Compile/GrammarToPGF.hs -2 +2
M ./src/compiler/GF/Grammar/Binary.hs -5 +1
M ./src/compiler/GF/Grammar/Lexer.x -11 +13
M ./src/compiler/GF/Infra/Ident.hs -19 +36
M ./src/runtime/haskell/PGF.hs -1 +1
M ./src/runtime/haskell/PGF/CId.hs -2 +3

src/compiler/GF/Compile/CheckGrammar.hs

@@ -298,9 +298,9 @@ checkInfo opts sgr (m,mo) c info = do
 
 -- | for grammars obtained otherwise than by parsing ---- update!!
 checkReservedId :: Ident -> Check ()
-checkReservedId x
-  | isReservedWord (ident2bs x) = checkWarn (text "reserved word used as identifier:" <+> ppIdent x)
-  | otherwise                   = return ()
+checkReservedId x =
+  when (isReservedWord x) $
+       checkWarn (text "reserved word used as identifier:" <+> ppIdent x)
 
 -- auxiliaries