suggestions

[jfcg]

• u dont need length field, same as len(frames)
• nice to have Prev(), like Next()
• i would use "if pos>=len {pos=0}" instead of modular arithmetic
• i would use mutexes sparingly, just inform user it is not multi-goroutine safe..

[atakanyenel]

Is it better to define spinner literals and default as const ? No impact on the code whatsoever, but seems like a better practice.