ENH: improve performance of df.to_csv GH3054
ENH: to_csv using masking to simplify dtype processing
ENH: more perf tweaks in df.to_csv
PERF: cythonized parts of to_csv for increased perf
PERF: more cython tweaks
PERF: cythonize improved python version
PERF: cythonized parts of to_csv for increased perf
PERF: more speedups
ENH: add chunksize parameter to DataFrame.to_csv to enable constant memory usage
     by writing in chunks
CLN: move repeated cast out of loop
CLN: make guard more defensive
REF: add com._ndarray_to_native_types
REF: apply native type conv to ix, cols before write_csv
PERF: added frame_to_csv2 vbench, revised frame_to_csv_mixed
TST: test for to_csv on failing vbench

duplicate column names across dtypes is a problem, and not-easy to fix,
so letting test fail .
ENH: refactor series from dict to list, eliminate one level of indirection
ENH: replace variable lookup by constant .

this is getting silly.
ENH: make chunks process constant element count
PERF: avoid iteritems->iloc panelty for data conversion, use blocks
TST: test for to_csv on failing vbench
     duplicate column names across dtypes is a problem, and
     not-easy to fix, so letting test fail