Better graphviz debugging (include values)

Dependencies

• [2] T7QB6QEP Adding debug.rs
• [3] OFINGD26 implementing prev() on cursors (+ some cleanup)

Change contents

• replacement in sanakirja/src/debug.rs at line 105
  [2.2481]→[2.2481:2547](∅→∅)
  − while let Some((key, _, r)) = P::next(txn, *p, &mut cursor) {
  [2.2481]

  [2.2547]
  + while let Some((key, val, r)) = P::next(txn, *p, &mut cursor) {
• replacement in sanakirja/src/debug.rs at line 117
  [2.2802]→[2.2802:2887](∅→∅)
  − writeln!(buf, "n_{}_{}[label=\"{}: {:?}\"];", p.offset, i, i, key).unwrap();
  [2.2802]

  [2.2887]
  + writeln!(

  + buf,

  + "n_{}_{}[label=\"{}: {:?} -> {:?}\"];",

  + p.offset, i, i, key, val

  + )

  + .unwrap();