improve output efficiency: use Text.PrettyPrint.Leijen instead of String

Hi. 

I needed to output some medium/large constraint systems,
and this function is definitely a performance killer:

group a = "( " ++ intercalate " " a ++ " )"

So I replaced String by Text.PrettyPrint.Leijen.Doc (after I tried with Text.PrettyPrint.HughesPJ which also had problems printing some large documents)

Best, J.W.