Refactor FIT2CSV

• Add Activities struct to hold multiple Activity entries
  • Moved the export_sessions_csv function to here
• Refactor types.rs into a folder with each type in its own file
  • Moved the relevant export functions from exporters.rs to activity.rs
    • Export session detail JSON
    • Export laps CSV
    • Export records CSV

No new functionality was introduced.

Full Changelog: v0.3.1...v0.3.2