2013-12-07-20.48.21: Much re-plumbing to provide  maybe_prettyprint_machcode_controlflow_graph()  src/lib/compiler/back/low/mcg/print-machcode-controlflow-graph-g.pkg  with access to  prettyprinter_or_null.  This suppresses the obnoxious assembly code the compiler has been spewing to the console since the prettyprinter rewrite disrupted things, and also takes us one step closer to proper harmonious integration between the backend upper and lower halves.