Simplifies `JoinKeys` from R6 to S3 list-like object #477

All 419 tests pass, 1 skipped in 14s

      1 files ±0     32 suites ±0 14s ⏱️ -1s
  420 tests ±0   419 ✔️ ±0 1 💤 ±0 0 ❌ ±0
1 023 runs ±0 1 022 ✔️ ±0 1 💤 ±0 0 ❌ ±0

Results for commit d72b9b0. ± Comparison against earlier commit ef6832b.