perf: some small refactors to improve perf

[latonis]

I think there is a breaking change coming with a refactor, but not yet :)

[github-actions]

Benchmark Results

flatten_inputs/default/small
                        time:   [1.0137 µs 1.0152 µs 1.0169 µs]
                        thrpt:  [983.41 Kelem/s 985.06 Kelem/s 986.53 Kelem/s]
                 change:
                        time:   [-0.9682% -0.4210% +0.0174%] (p = 0.10 > 0.05)
                        thrpt:  [-0.0174% +0.4227% +0.9777%]
flatten_inputs/preserve_arrays/small
                        time:   [1.1054 µs 1.1070 µs 1.1086 µs]
                        thrpt:  [902.00 Kelem/s 903.38 Kelem/s 904.68 Kelem/s]
                 change:
                        time:   [-2.0995% -1.7154% -1.4151%] (p = 0.00 < 0.05)
                        thrpt:  [+1.4354% +1.7453% +2.1445%]
                        Performance has improved.
flatten_inputs/alt_array_flattening/small
                        time:   [1.1019 µs 1.1042 µs 1.1067 µs]
                        thrpt:  [903.56 Kelem/s 905.66 Kelem/s 907.53 Kelem/s]
                 change:
                        time:   [-2.4374% -2.0841% -1.8029%] (p = 0.00 < 0.05)
                        thrpt:  [+1.8360% +2.1284% +2.4983%]
                        Performance has improved.
flatten_inputs/default/medium
                        time:   [49.005 µs 49.043 µs 49.081 µs]
                        thrpt:  [20.375 Kelem/s 20.390 Kelem/s 20.406 Kelem/s]
                 change:
                        time:   [+1.3593% +1.6275% +1.8035%] (p = 0.00 < 0.05)
                        thrpt:  [-1.7716% -1.6015% -1.3410%]
                        Performance has regressed.
flatten_inputs/preserve_arrays/medium
                        time:   [47.405 µs 47.442 µs 47.480 µs]
                        thrpt:  [21.061 Kelem/s 21.079 Kelem/s 21.095 Kelem/s]
                 change:
                        time:   [-1.9810% -1.8121% -1.6580%] (p = 0.00 < 0.05)
                        thrpt:  [+1.6860% +1.8455% +2.0210%]
                        Performance has improved.
flatten_inputs/alt_array_flattening/medium
                        time:   [49.620 µs 49.658 µs 49.700 µs]
                        thrpt:  [20.121 Kelem/s 20.138 Kelem/s 20.153 Kelem/s]
                 change:
                        time:   [+2.3959% +2.6394% +2.8310%] (p = 0.00 < 0.05)
                        thrpt:  [-2.7531% -2.5716% -2.3399%]
                        Performance has regressed.
flatten_inputs/default/large
                        time:   [1.1332 ms 1.1360 ms 1.1382 ms]
                        thrpt:  [878.55  elem/s 880.28  elem/s 882.46  elem/s]
                 change:
                        time:   [+9.7852% +10.479% +11.050%] (p = 0.00 < 0.05)
                        thrpt:  [-9.9506% -9.4847% -8.9131%]
                        Performance has regressed.
flatten_inputs/preserve_arrays/large
                        time:   [1.1076 ms 1.1088 ms 1.1104 ms]
                        thrpt:  [900.58  elem/s 901.85  elem/s 902.87  elem/s]
                 change:
                        time:   [+8.7426% +9.1281% +9.4124%] (p = 0.00 < 0.05)
                        thrpt:  [-8.6027% -8.3646% -8.0397%]
                        Performance has regressed.
flatten_inputs/alt_array_flattening/large
                        time:   [1.1177 ms 1.1186 ms 1.1195 ms]
                        thrpt:  [893.22  elem/s 893.96  elem/s 894.67  elem/s]
                 change:
                        time:   [+9.8832% +10.056% +10.229%] (p = 0.00 < 0.05)
                        thrpt:  [-9.2799% -9.1371% -8.9943%]
                        Performance has regressed.
flatten_collisions/collisions_default/10
                        time:   [1.6604 µs 1.6627 µs 1.6655 µs]
                        thrpt:  [600.42 Kelem/s 601.43 Kelem/s 602.26 Kelem/s]
                 change:
                        time:   [-5.7848% -5.5019% -5.2154%] (p = 0.00 < 0.05)
                        thrpt:  [+5.5023% +5.8222% +6.1400%]
                        Performance has improved.
flatten_collisions/collisions_alt_flatten/10
                        time:   [1.6193 µs 1.6205 µs 1.6217 µs]
                        thrpt:  [616.63 Kelem/s 617.11 Kelem/s 617.56 Kelem/s]
                 change:
                        time:   [-5.1891% -4.9606% -4.7898%] (p = 0.00 < 0.05)
                        thrpt:  [+5.0308% +5.2195% +5.4730%]
                        Performance has improved.
flatten_collisions/collisions_default/100
                        time:   [14.704 µs 14.746 µs 14.797 µs]
                        thrpt:  [67.579 Kelem/s 67.815 Kelem/s 68.008 Kelem/s]
                 change:
                        time:   [-4.8086% -4.3075% -3.8418%] (p = 0.00 < 0.05)
                        thrpt:  [+3.9953% +4.5015% +5.0515%]
                        Performance has improved.
flatten_collisions/collisions_alt_flatten/100
                        time:   [15.093 µs 15.102 µs 15.112 µs]
                        thrpt:  [66.174 Kelem/s 66.217 Kelem/s 66.255 Kelem/s]
                 change:
                        time:   [-3.0518% -2.8329% -2.6499%] (p = 0.00 < 0.05)
                        thrpt:  [+2.7221% +2.9155% +3.1479%]
                        Performance has improved.
flatten_collisions/collisions_default/1000
                        time:   [139.76 µs 140.15 µs 140.62 µs]
                        thrpt:  [7.1112 Kelem/s 7.1350 Kelem/s 7.1554 Kelem/s]
                 change:
                        time:   [-3.4281% -2.5766% -1.8046%] (p = 0.00 < 0.05)
                        thrpt:  [+1.8378% +2.6448% +3.5498%]
                        Performance has improved.
flatten_collisions/collisions_alt_flatten/1000
                        time:   [141.73 µs 141.92 µs 142.10 µs]
                        thrpt:  [7.0373 Kelem/s 7.0464 Kelem/s 7.0558 Kelem/s]
                 change:
                        time:   [-1.9190% -1.6944% -1.4909%] (p = 0.00 < 0.05)
                        thrpt:  [+1.5134% +1.7236% +1.9566%]
                        Performance has improved.

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Benchmarked on commit 4c9aaef92eced0f2eb5bf3a59d08b39ed321ef51