Refactor internals module part 4

[pitdicker]

These commits add a way to construct an Mdf without going through an Of, and to get an ordinal and flags back out without going through Of. With that the last uses of Of are gone.

A final commit is to move cycle_to_yo and yo_to_cycle and the related array to the date module.

I found one comment in a backup branch that was better to have with the previous PR, sorry. Just stylistic comment changes to IsoWeek.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7c4b119) 91.84% compared to head (ae14eb7) 91.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1432      +/-   ##
==========================================
+ Coverage   91.84%   91.87%   +0.02%     
==========================================
  Files          38       38              
  Lines       17571    17507      -64     
==========================================
- Hits        16138    16084      -54     
+ Misses       1433     1423      -10     

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