[Overview] Improve existing Concept Exercises

This is a tracking issue for the concept exercises that need some cleanup or improvement.

• https://github.com/exercism/go/issues/1936 (claimed)
• https://github.com/exercism/go/issues/1755
• https://github.com/exercism/go/issues/1731
• https://github.com/exercism/go/issues/1583 & https://github.com/exercism/go/issues/1584
• https://github.com/exercism/go/issues/1579
• https://github.com/exercism/go/issues/1574
• https://github.com/exercism/go/issues/1990
• https://github.com/exercism/go/issues/1565

There are probably some more issues needed, feel free to create additional ones if you notice problems with existing concept exercises.