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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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- [ ] 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
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