Unsized pages: decrement n on deletions

Dependencies

• [2] Q7DRIBBR Debugging replace (which cannot be del+put)
• [3] H3FVSQIQ Unsized pages

Change contents

• replacement in sanakirja-core/src/btree/page_unsized/put.rs at line 51
  [2.3967]→[2.3967:4016](∅→∅)
  − header_mut(&mut page).decr(size)
  [2.3967]

  [2.4016]
  + let hdr = header_mut(&mut page);

  + hdr.decr(size);

  + hdr.set_n(hdr.n() - 1);