Simplifies `JoinKeys` from R6 to S3 list-like object #504

All 423 tests pass, 1 skipped in 15s

      1 files ±0     36 suites ±0 15s ⏱️ ±0s
  424 tests ±0   423 ✔️ ±0 1 💤 ±0 0 ❌ ±0
1 027 runs ±0 1 026 ✔️ ±0 1 💤 ±0 0 ❌ ±0

Results for commit 469bc5a. ± Comparison against earlier commit e525093.