feat(flow): implement rolling fold transformation

[Wkkkkk]

Motivation

• A fold flow can be useful in building up a state given the input stream. For example, one could use it to simulate walking in a one-dimensional maze.

Modifications

• flow/fold.go
• examples/maze/

Verify change

• Make sure the change passes the CI checks.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.

Project coverage is 99.18%. Comparing base (683f3f7) to head (90263a6).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
flow/fold.go	90.62%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
- Coverage   99.57%   99.18%   -0.39%     
==========================================
  Files          12       13       +1     
  Lines         699      739      +40     
==========================================
+ Hits          696      733      +37     
- Misses          3        6       +3     

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[reugn]

@Wkkkkk, thanks for the pull request. Please address linter warnings.

[Wkkkkk]

Hi @reugn, it is now fixed.