Simplifies `JoinKeys` from R6 to S3 list-like object #690

All 423 tests pass, 1 skipped in 15s

      1 files ±0     32 suites ±0 15s ⏱️ -1s
  424 tests ±0   423 ✔️ +1 1 💤 ±0 0 ❌ - 1
1 027 runs ±0 1 026 ✔️ +1 1 💤 ±0 0 ❌ - 1

Results for commit 53d1df6. ± Comparison against earlier commit 491ad61.