PrettyJSON: Implement 'oneChar :: Char -> Doc'

Implement 'oneChar :: Char -> Doc' and 'simpleEscapes :: [(Char, String)]'.
Stub out 'hexEscape :: Char -> Doc'.
PrettyJSON: Implement 'showHex :: Int -> Doc'
PrettyJSON: Implement 'astral :: Int -> Doc'
PrettyJSON: Implement 'hexEscape :: Char -> Doc'