When deleting in internal nodes, set the correct child

Dependencies

• [2] H3FVSQIQ Unsized pages
• [3] OTWDDJE7 Trait/type cleanup
• [4] OFINGD26 implementing prev() on cursors (+ some cleanup)
• [5] MSRWB47Y Deletions at immutable leaves weren't really deleting anything
• [6] OP6SVMOD Resetting history
• [7] KX3WVNZW Testing/debugging "rebalance causes split of the root"
• [8] WS4ZQM4R Debugging, tests, etc.

Change contents

• replacement in sanakirja-core/src/btree/page_unsized.rs at line 188
  [2.8398]→[2.8398:8436](∅→∅)
  − *off = l.to_le();
  [2.8398]

  [2.8436]
  + *off = (l | (u64::from_le(*off) & 0xfff)).to_le();
• replacement in sanakirja-core/src/btree/page.rs at line 203
  [3.15501]→[3.15501:15539](∅→∅)
  − *off = l.to_le();
  [3.15501]

  [3.2461]
  + *off = (l | (u64::from_le(*off) & 0xfff)).to_le();