Generalize pretty-printing onto rich streams

[shingarov]

PR #153 implements only the special case needed for Notarfrancesco-style printing of multiline things like Matrices (or in our case, Gentzen ND rules). In general, the check for whether the Stream is rich-textual, should not be specific to Form, should be double-dispatched to the stream, and this whole thing should be done up in Object.

[shingarov]

Hmm, I wonder how far one can go with it. What if aStream is some kind of Assembler Syntax stream — so now Tinyrossa could pipe Instructions into gas when AoT-ing simply by eachInstruction printOn: anAssemblyStream?