exercises/21

• homework/
• 01-square.go
• 02-divmod.go
• 03-counter-scope.go
• 04-outside-function.go
• 05-check-atoi.go
• 06-your-own-errors.go
• 07-read-until-end.go
• 08-read-functions.go
• 09-validate-volume.go
• 10-read-guess.go