Digits quasi quoter

Doctest fails to parse `TemplateHaskell`.

```hs
[digitQ|4|] :: Digit
-- 4

named [digitQ|4|]  = "four"
named [digitQ|$x|] = "not four, " ++ show x ++ " instead"

mod10D x = let y = mod x 10 in [digitQ|$y|]
```