Faster dask unstack

[Illviljan]

ref #5582

• Closes #xxxx
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[Illviljan]

Hmm, this way doesn't seem faster on first glance?

[ 72.78%] ··· unstacking.Unstacking.time_unstack_fast                1.91±0.01ms
[ 72.96%] ··· unstacking.Unstacking.time_unstack_pandas_slow          13.9±0.1ms
[ 73.15%] ··· unstacking.Unstacking.time_unstack_slow                1.91±0.02ms
[ 73.33%] ··· unstacking.UnstackingDask.time_unstack_fast            21.8±0.08ms
[ 73.52%] ··· ...king.UnstackingDask.time_unstack_pandas_slow         14.6±0.1ms
[ 73.70%] ··· unstacking.UnstackingDask.time_unstack_slow            1.94±0.02ms
[ 73.89%] ··· ...nstackingSparse.peakmem_unstack_to_sparse_2d               204M
[ 74.07%] ··· ...nstackingSparse.peakmem_unstack_to_sparse_3d               204M
[ 74.26%] ··· unstacking.UnstackingSparse.time_unstack_fast           16.4±0.2ms
[ 74.44%] ··· ...ng.UnstackingSparse.time_unstack_pandas_slow         58.3±0.2ns
[ 74.63%] ··· unstacking.UnstackingSparse.time_unstack_slow              1.20±0s
[ 74.81%] ··· ...g.UnstackingSparse.time_unstack_to_sparse_2d            444±5μs
[ 75.00%] ··· ...g.UnstackingSparse.time_unstack_to_sparse_3d        4.36±0.02ms

[ 97.78%] ··· unstacking.Unstacking.time_unstack_fast                1.85±0.09ms
[ 97.96%] ··· unstacking.Unstacking.time_unstack_pandas_slow          13.2±0.6ms
[ 98.15%] ··· unstacking.Unstacking.time_unstack_slow                   3.80±2ms
[ 98.33%] ··· unstacking.UnstackingDask.time_unstack_fast              40.7±20ms
[ 98.52%] ··· ...king.UnstackingDask.time_unstack_pandas_slow          25.2±10ms
[ 98.70%] ··· unstacking.UnstackingDask.time_unstack_slow               4.00±2ms
[ 98.89%] ··· ...nstackingSparse.peakmem_unstack_to_sparse_2d               204M
[ 99.07%] ··· ...nstackingSparse.peakmem_unstack_to_sparse_3d               204M
[ 99.26%] ··· unstacking.UnstackingSparse.time_unstack_fast            28.1±10ms
[ 99.44%] ··· ...ng.UnstackingSparse.time_unstack_pandas_slow           113±60ns
+ grep 'Traceback \|failed\|PERFORMANCE DECREASED' benchmarks.log
[ 99.63%] ··· unstacking.UnstackingSparse.time_unstack_slow            1.09±0.6s
[ 99.81%] ··· ...g.UnstackingSparse.time_unstack_to_sparse_2d          864±400μs
[100.00%] ··· ...g.UnstackingSparse.time_unstack_to_sparse_3d           8.56±4ms

[dcherian]

ravel_multi_index can be slow. Maybe that's it?