lib/richPp: Fix a bug related to the compatibility with ocaml 3.12
  - The previous version of this module was using a feature of
    the Format module of ocaml 4.01.

  - Add comments.
lib/RichPp: Rename into Richpp.
printing/RichPrinter: Rename into Richprinter.
printing/{ppvernac, ppconstr, pptactic}: Rename RichPp into Richpp.
printing/Richprinter: Cosmetics.