ENH: memory_map for compressed files
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Apply memory mapping first and then compression. The existing order didn't work.

This adds support for memory mapping in `read_csv` when compression is specified (python and c engine).