0.5.6 — drop the printers crate (no CUPS build dep)

Changed

• rsplot: the toolbar Print action no longer depends on the printers
  crate, so a default build links no CUPS library (-lcups) and needs no
  libcups2-dev at build time. Printer enumeration and job submission now shell
  out to the lpstat / lp command-line tools. Printing works wherever those
  are on PATH (CUPS on Linux/macOS); on platforms without them (e.g. Windows)
  the printer list is empty and the print action reports no destination
  (print_graph returns Ok(false)). The public print_graph /
  print_graph_to signatures are unchanged.

Removed

• rsplot: the printers crate dependency (and its transitive CUPS /
  libcups build requirement).