Simplifies `JoinKeys` from R6 to S3 list-like object #437

All 410 tests pass, 1 skipped in 14s

      1 files ±0     32 suites ±0 14s ⏱️ -1s
  411 tests ±0   410 ✔️ ±0 1 💤 ±0 0 ❌ ±0
1 011 runs ±0 1 010 ✔️ ±0 1 💤 ±0 0 ❌ ±0

Results for commit 27d5ff3. ± Comparison against earlier commit 9f0fdc2.