Refactor

[koenichiwa]

• Pulled ScheduleFields and Field to their own file.
• Pulled OrdinalIter and OrdinalRangeIter to the ordinal.rs file.
  • in the end I decided that it might be better to change it's implementation to keep it clean. I discussed this already in #82. Honestly I think that the old implementation mightve been one of the problems causing the slowdown for #80
• Put trait implementations in their respective files.

[zslayton]

Closing for now, feel free to reopen.