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
CLN: refactor of csv
     moved code to fmt/CSVFormatter
     moved block and index formatting code to internals/index