PERF: Slowness in multi-level indexes with datetime levels

Special case handling of sliced multi indexes, where there the length of the level values may exceed the length of the index.
Corrected nan handling issue introduced by the original change. (+1 squashed commit)
Squashed commits:
[0dae170] PERF: Slowness in multi-level indexes with datetime levels

Special case handling of sliced multi indexes, where there the length of the level values may exceed the length of the index.
PERF: fixup datetime_index repr perf (GH8556)
Merge pull request #8557 from jreback/dt_perf

PERF: fixup datetime_index repr perf (GH8556)
DOC: formatting issue in scatter_matrix section
DOC/FIX: In R it's still levels...
Merge pull request #8561 from JanSchulz/r_levels

DOC/FIX: In R it's still levels...
BUG: _Openpyxl2Writer mishandles merged cell style

TST: Cover merged cells case in openpyxl2 engine
Merge pull request #8549 from mtkni/master

PERF: Slowness in multi-level indexes with datetime levels (part 2)
DOC: ignore_na warning
Switch default colormap for scatterplot to 'Greys'

In #7780, I added colormap support to scatter, but choose to override the
default rainbow colormap (7780#issuecomment-49533995).

I'm now regreting choosing 'RdBu' as the default colormap. I think a simple
black-white colormap is a more conservative and better default choice for
coloring scatter plots -- binary colormaps really only make sense if the data
is signed. 'Greys' is also the default colormap set by the seaborn package,
and @mwaskom has done far more thinking about colormaps than I have.

If possible, I would love to get this in before 0.15 is released, so we don't
have to worry about changing how anyone's plots look.

CC @sinhrks
Merge pull request #8567 from seth-p/ignore_na_warning

DOC: ignore_na warning
Merge pull request #8566 from shoyer/fix-default-cmap

Switch default colormap for scatterplot to 'Greys'
PERF: improve perf of array_equivalent_object (GH8512)
Merge pull request #8570 from jreback/equals

PERF: improve perf of array_equivalent_object (GH8512)
ENH: Add type conversion to read_stata and StataReader

Added upcasting of numeric types which will default numeric data to either
int64 or float64.  This option has been enabled by default.

closes #8527
Merge pull request #8564 from bashtage/stata-datatype-conversion

ENH: Add type conversion to read_stata and StataReader
TST/PERF: optimize tm.makeStringIndex

* add tm.rands_array(nchars, size) function to generate string arrays

* add tm.randu_array(nchars, size) function to generate unicode arrays

* replace [rands(N) for _ in range(X)] idioms:
  - with makeStringIndex in benchmarks to maintain backward compatibility
  - with rands_array in tests to maintain 1-to-1 type correspondence