Fully using new workspace

[patrickhyw]

Summary: The codebase is now fully refactored to use the new open_and_save(), save_file(), and link_result() functions from #66. The old functions have been removed.

Demo:
Wrote two new integration test files that pass.

Details

• Refactored dbms/ and benchmark/ to use the new files. Wrote tests for both too.
• Moved path functions from workspace.py to gymlib/symlinks_paths.py.
  • This is important because agents will need access to the DBMS/benchmark paths.
• Renamed all occurrences of dpath/fpath/fordpath to path. Renamed dname/fname to dirname/filename.
  • Names could refer to conceptual names so we add dir/file to the front to disambiguate.
  • Paths are not ambiguous though so we just call them paths. Whether they're a directory or file matters a little, but I think it's cleaner to just call them paths since that's the general standard.