Merge branch 'GH3054/to_csv_perf'

* GH3054/to_csv_perf: (28 commits)
  TST: mark csv test @slow
  DOC: update what's new, RELEASE.rst
  BUG: MultiIndex to_native_types did not obey slicer
  TST: test the hell out of the new df.to_csv()
  CLN: preallocate data array only once
  TST: fail early on duplicate columns
  ENH: add (undocumented) legacy kwd to df.to_csv, just in case
  CLN: csv refactor
  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
  PERF: avoid iteritems->iloc panelty for data conversion, use blocks
  ENH: make chunks process constant element count
  ENH: replace variable lookup by constant .
  ENH: refactor series from dict to list, eliminate one level of indirection
  TST: test for to_csv on failing vbench
  PERF: added frame_to_csv2 vbench, revised frame_to_csv_mixed
  REF: apply native type conv to ix, cols before write_csv
  REF: add com._ndarray_to_native_types
  CLN: make guard more defensive
  CLN: move repeated cast out of loop
  ENH: add chunksize parameter to DataFrame.to_csv to enable constant memory usage      by writing in chunks
  ...

Conflicts:
	RELEASE.rst
	doc/source/v0.11.0.txt
	pandas/core/internals.py