identifiers in Text.Parsec.Language not polymorphic enough

we have

```
emptyDef :: LanguageDef st
type LanguageDef st = GenLanguageDef String st Identity
makeTokenParser :: Stream s m Char => GenLanguageDef s u m -> GenTokenParser s u m
```

This means that this can only be used for `String` parsers, and the following is ill-typed (whatever the contents of `{ ... }`

```
lexer :: GenTokenParser ByteString () Identity
lexer = makeTokenParser $ emptyDef { ... }
```