Change how GF deals with character encodings in grammar files

1. The default encoding is changed from Latin-1 to UTF-8.

2. Alternate encodings should be specified as "--# -coding=enc", the old
   "flags coding=enc" declarations have no effect but are still checked for
   consistency.

3. A transitional warning is generated for files that contain non-ASCII
   characters without specifying a character encoding:

	"Warning: default encoding has changed from Latin-1 to UTF-8"

4. Conversion to Unicode is now done *before* lexing. This makes it possible
   to allow arbitrary Unicode characters in identifiers. But identifiers are
   still stored as ByteStrings, so they are limited to Latin-1 characters
   for now.

5. Lexer.hs is no longer part of the repository. We now generate the lexer
   from Lexer.x with alex>=3. Some workarounds for bugs in alex-3.0 were
   needed. These bugs might already be fixed in newer versions of alex, but
   we should be compatible with what is shipped in the Haskell Platform.

src/compiler/GF/Grammar/Parser.y

@@ -17,8 +17,8 @@ import GF.Grammar.Grammar
 import GF.Grammar.Macros
 import GF.Grammar.Lexer
 import GF.Compile.Update (buildAnyTree)
-import Codec.Binary.UTF8.String(decodeString)
-import Data.Char(toLower)
+--import Codec.Binary.UTF8.String(decodeString)
+--import Data.Char(toLower)
 }
 
 %name pModDef ModDef
@@ -616,9 +616,9 @@ happyError = fail "syntax error"
 
 -- Quick fix to render error messages from UTF-8-encoded source files correctly.
 optDecode opts =
-    if map toLower (flag optEncoding opts) `elem` ["utf8","utf-8"]
+    {-if map toLower (getEncoding opts) `elem` ["utf8","utf-8"]
     then decodeString
-    else id
+    else-} id
 
 mkListId,mkConsId,mkBaseId  :: Ident -> Ident
 mkListId = prefixIdent "List"