ENH: Allow unicode separators in to_csv & read_csv

* unicode delimiters coercible to ASCII (e.g., `u(',')`) are always allowed.
* unicode delimiters that don't fit within ASCII are always disallowed
  with C engine.
* unicode delimiters that don't fit within ASCII will never work in
  Python 2 engines (because their CSV assumes seps are unicode).

Closes #6035.