Simplifies `JoinKeys` from R6 to S3 list-like object #475

All 419 tests pass, 1 skipped in 14s

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

Results for commit dc6df6a. ± Comparison against earlier commit 26eff1b.