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## Description
Speedup coins.String() and coin.String()
In my local benchmarks, this >2x improves the time for balances with 1 coin, and further improves speed for strings with many balances. I did not benchmark on the two coin usecase
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### Author Checklist
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I have...
- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
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- [x] provided a link to the relevant issue or specification - n/a
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- [ ] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc) - seems sufficiently clear
- [x] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed
### Reviewers Checklist
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- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)
Dev Ojha
744c85b8fa