Structure of the new lexer

The implementation consists of the following key ingredients: 
program       :: RegExp HaskellChar Token
lexerGen      :: RegExp HaskellChar Token -> HaskellSourceCode

haskellLex    :: String -> [(Token,String)]
nestedComment :: ...

addPos    :: [(Token,String)] -> [(Token,(Pos,String))]
rmSpace   :: [(Token,(Pos,String))] -> [(Token,(Pos,String))]
layoutPre :: [(Token,(Pos,String))] -> [(Token,(Pos,String))]

token     :: PM (Token,(Pos,String))
popContext:: PM ()

parseFile :: PM a -> FilePath -> String -> Either Error a
See modules HsLexerPass1, HsLexer, HsLexUtils.