PERF: unstack

[jbrockmendel]

from asv_bench.benchmarks.reshape import *
self = Unstack()
self.setup("category")

%timeit self.time_without_last_row("category")
77.3 ms ± 1.97 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)  # <- master
21.2 ms ± 788 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)  # <- PR

[jreback]

hmm can you avoid repeating this? (e.g. could create a needs_masking[i] with all values == False for the not allow_fill case

[jbrockmendel]

sure

[jreback]

nice!