currency: add function FindMatchingPairs

[shazbert]

PR Description

This is just cherry-picked from an external branch if any body wants it. Can close if not.

Fixes # (issue)

Type of change

Please delete options that are not relevant and add an x in [] as item is complete.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How has this been tested

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration and
also consider improving test coverage whilst working on a certain feature or package.

• go test ./... -race
• golangci-lint run
• Test X

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation and regenerated documentation via the documentation tool
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally and on Github Actions with my changes
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

Attention: Patch coverage is 85.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 36.03%. Comparing base (f1ff951) to head (67e3393).
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1531      +/-   ##
==========================================
- Coverage   36.03%   36.03%   -0.01%     
==========================================
  Files         411      411              
  Lines      177750   177770      +20     
==========================================
- Hits        64061    64051      -10     
- Misses     105839   105864      +25     
- Partials     7850     7855       +5     

Files	Coverage Δ
currency/pairs.go	97.25% <85.00%> (-0.80%)	⬇️

... and 16 files with indirect coverage changes