feat: New last_rel() to retrieve the last relation object used in materialization

[krlmlr]

Closes #209.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 6699c2d is merged into main:

• ✔️001_tpch_01: 24.3ms -> 23.8ms [-5.86%, +2.5%]
• ✔️001_tpch_02: 121ms -> 122ms [-0.99%, +2.4%]
• ✔️001_tpch_03: 62.8ms -> 62.5ms [-1.68%, +0.71%]
• ✔️001_tpch_04: 22.8ms -> 22.8ms [-2.52%, +2.53%]
• ✔️001_tpch_05: 117ms -> 116ms [-2.65%, +0.53%]
• ✔️001_tpch_06: 14.8ms -> 14.4ms [-5.73%, +0.44%]
• ✔️001_tpch_07: 133ms -> 133ms [-1.2%, +0.77%]
• ✔️001_tpch_08: 152ms -> 153ms [-0.31%, +1.39%]
• ✔️001_tpch_09: 126ms -> 128ms [-1.14%, +3.35%]
• ✔️001_tpch_10: 72ms -> 72.1ms [-1.17%, +1.39%]
• ✔️001_tpch_11: 59.7ms -> 60.1ms [-1.25%, +2.6%]
• ✔️001_tpch_12: 27.7ms -> 27.6ms [-1.43%, +0.31%]
• ✔️001_tpch_13: 19.5ms -> 19.6ms [-1.17%, +2.42%]
• ✔️001_tpch_14: 20.6ms -> 20.5ms [-1.81%, +0.39%]
• ✔️001_tpch_15: 60.8ms -> 60.6ms [-2.21%, +1.39%]
• ✔️001_tpch_16: 62.9ms -> 62.4ms [-2.39%, +0.58%]
• ✔️001_tpch_17: 26.8ms -> 26.4ms [-3.38%, +0.21%]
• ✔️001_tpch_18: 23ms -> 22.8ms [-2.57%, +0.85%]
• ✔️001_tpch_19: 124ms -> 125ms [-0.8%, +1.79%]
• ✔️001_tpch_20: 76.1ms -> 77.1ms [-0.99%, +3.58%]
• ✔️001_tpch_21: 168ms -> 168ms [-1.63%, +1.38%]
• ✔️001_tpch_22: 121ms -> 120ms [-1.63%, +0.79%]
• ✔️010_tpch_01: 82.3ms -> 87.7ms [-1.81%, +14.78%]
• ✔️010_tpch_02: 72.6ms -> 71.4ms [-4.59%, +1.38%]
• ✔️010_tpch_03: 64.6ms -> 65.3ms [-0.85%, +3.07%]
• ✔️010_tpch_04: 46.1ms -> 46.4ms [-1.99%, +3.25%]
• ✔️010_tpch_05: 93.8ms -> 93.7ms [-1.33%, +1.24%]
• ✔️010_tpch_06: 34.3ms -> 36.8ms [-1.75%, +16.5%]
• ✔️010_tpch_07: 109ms -> 111ms [-0.74%, +3.4%]
• ✔️010_tpch_08: 131ms -> 131ms [-1.95%, +2.14%]
• ✔️010_tpch_09: 119ms -> 120ms [-2.17%, +4.07%]
• ✔️010_tpch_10: 78.9ms -> 79.1ms [-1.14%, +1.57%]
• ✔️010_tpch_11: 37.6ms -> 38.2ms [-2.67%, +5.65%]
• ✔️010_tpch_12: 60.2ms -> 60ms [-1.19%, +0.59%]
• ✔️010_tpch_13: 53.3ms -> 53.3ms [-2.59%, +2.68%]
• ✔️010_tpch_14: 42.5ms -> 41.4ms [-7.5%, +2.21%]
• ✔️010_tpch_15: 59.7ms -> 60.1ms [-4.65%, +6.15%]
• ✔️010_tpch_16: 44.9ms -> 45.1ms [-2.46%, +3.59%]
• ✔️010_tpch_17: 55.5ms -> 57.1ms [-0.55%, +6.28%]
• ✔️010_tpch_18: 55.7ms -> 57ms [-4.56%, +9.38%]
• 🚀010_tpch_19: 118ms -> 116ms [-2.56%, -0.64%]
• ✔️010_tpch_20: 70.1ms -> 69.2ms [-3.76%, +1.14%]
• ✔️010_tpch_21: 374ms -> 373ms [-1.64%, +1.1%]
• ✔️010_tpch_22: 75.9ms -> 77.1ms [-1.02%, +4.06%]
• ✔️100_tpch_01: 1.16s -> 1.16s [-2.52%, +0.97%]
• ✔️100_tpch_02: 131ms -> 126ms [-10.42%, +2.43%]
• ✔️100_tpch_03: 1.04s -> 1.03s [-2.39%, +0.8%]
• ✔️100_tpch_04: 1.01s -> 1.02s [-2.91%, +3.3%]
• ✔️100_tpch_05: 1.14s -> 1.12s [-4.69%, +1.2%]
• ✔️100_tpch_06: 963ms -> 965ms [-0.96%, +1.47%]
• ✔️100_tpch_07: 1.09s -> 1.09s [-3.19%, +2.33%]
• ✔️100_tpch_08: 1.11s -> 1.11s [-2.15%, +1.97%]
• ✔️100_tpch_09: 1.21s -> 1.21s [-3.85%, +4.73%]
• ✔️100_tpch_10: 1.07s -> 1.06s [-1.09%, +0.2%]
• ✔️100_tpch_11: 84.6ms -> 82.8ms [-9%, +4.89%]
• ✔️100_tpch_12: 1.06s -> 1.05s [-3.84%, +2.19%]
• ✔️100_tpch_13: 320ms -> 324ms [-3.34%, +6.16%]
• ✔️100_tpch_14: 982ms -> 983ms [-2.91%, +3.09%]
• ✔️100_tpch_15: 1.07s -> 1.06s [-3.77%, +1.97%]
• ✔️100_tpch_16: 124ms -> 128ms [-9.74%, +15.41%]
• ✔️100_tpch_17: 1.04s -> 1.05s [-2.84%, +4.57%]
• ✔️100_tpch_18: 1.07s -> 1.07s [-2.27%, +2.88%]
• ✔️100_tpch_19: 1.13s -> 1.13s [-0.59%, +0.93%]
• ✔️100_tpch_20: 1.04s -> 1.03s [-2.74%, +1.91%]
• ✔️100_tpch_21: 2.23s -> 2.17s [-9.34%, +3.39%]
• ✔️100_tpch_22: 173ms -> 168ms [-5.88%, +0.21%]

Further explanation regarding interpretation and methodology can be found in the documentation.