Implement MapUp and MapDown

[BooleanCat]

Please provide a brief description of the change.

Implement MapUp and MapDown to convert between iter.Seq and iter.Seq2.

Which issue does this change relate to?

None.

Contribution checklist.

• I have read and understood the CONTRIBUTING guidelines
• All commits in my PR conform to the commit hygiene section
• I have added relevant tests
• I have not added any dependencies

Additional context

None.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (48b826e) to head (2ef35ac).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #157   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           34        34           
  Lines          624       636   +12     
=========================================
+ Hits           624       636   +12     

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