Pivot performance

[flexatone]

Sample performance: SF only outperforms in 1 of 4 benchmarks.

2024-02-05 16:37:45.405771
python:3.11.1|numpy:1.25.2|pandas:2.0.3|static_frame:2.2.3
name                                   iterations Native Reference n/r    r/n    win              status       explanation
Pivot.index1_columns0_data2 150.0      0.5046 0.748     0.6746 1.4824 True             ✓            
Pivot.index1_columns1_data1 150.0      2.4184 1.5168    1.5944 0.6272 False ✓ 
Pivot.index1_columns1_data3 150.0      2.5669 1.9334    1.3277 0.7532 False ✓            
Pivot.index2_columns0_data1 150.0      2.3125 1.4134    1.6362 0.6112 False ✓ 

[flexatone]

PR #900 introduced many minor improvements but did not show a material benefit. Other approaches will be explored under a different PR.