PERF: DataFrame.insert

[jbrockmendel]

• closes #xxxx
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

from asv_bench.benchmarks.indexing import *

self = InsertColumns()
self.setup()

%timeit self.setup(); self.time_insert()
32.5 ms ± 382 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)   # <- master
20.7 ms ± 744 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)   # <- PR

%timeit self.setup(); self.time_insert_middle()
37.2 ms ± 407 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)  # <- master
25.1 ms ± 513 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)  # <- PR

[jreback]

looks fine. can you rebase