silkscreen: prettyprinting transformers

🎩 @3_lemma for the name!

Silkscreen abstracts the pretty-printing interface of prettyprinter with a Printer typeclass, allowing the definition of composable pretty-printer transformers, layering new behaviours onto the existing primitives. For example, Silkscreen.Prec implements support for precedence à la showsPrec, but using symbolic precedence levels, and Silkscreen.Rainbow implements rainbow-parentheses.