improving df.to_csv performance
This is a WIP and discuss PR :construction: for #3186 
Adding small Writer class to have short-lived self.writer in CSVFomatter just for the header line. It is closed after the header was written.
Adding new lib.pyx function write_csv_rows_fast that manages file open/close on its own with self.path_or_buf from CSVFormmatter.
Current speed-up is factor 2.5 compared to before. Possibly the fprintf for each item is inefficient, but I thought as fprintf is buffered (in contrast to write) it can't be too bad. Any suggestions what I'm doing horribly wrong?

closing for now. pls reopen if you want to update.
