Exercises for #48in24

Dear maintainers,

Jeremy posted an overview of the exercises that will be featured in #48in24:
http://forum.exercism.org/t/which-exercises-for-48in24/8970/19

The following featured exercises have not been translated to Nim yet:

- [ ] parallel-letter-frequency
- [ ] zebra-puzzle
- [ ] minesweeper
- [ ] pig-latin
- [ ] robot-simulator (@ilmanzo, https://github.com/exercism/nim/pull/558)
- [ ] knapsack (@siebenschlaefer, #563)
- [ ] two-bucket (@siebenschlaefer, #566)
- [ ] food-chain
- [ ] bank-account
- [ ] circular-buffer
- [ ] pop-count

I'm posting them here as a reminder.
I hope I will find the time to translate a few of them (although I'm rather a beginner in Nim), and perhaps others will help, too.
Feel free to close or remove this issue if you do not find it helpful.

Edit:
There is now a dedicated matrix with the status of all exercises in #48in24, which tracks have implemented them, and which exercises are featured on a specific track:
https://exercism.org/challenges/48in24/implementation_status

Review the listed #48in24 exercises and the linked implementation-status matrix to choose one that is still missing from the Nim track. Check existing Nim exercise conventions before starting; the issue is done when the selected exercise has been translated and its track status is reflected in the matrix.

Written by the indexing model from the issue text.