Switch API to ReadByte, WriteByte, and Reset.

Leaving everything else in the test more or less as-is, change the API for the reason given in the comment. No one else has submitted a solution to this exercise in Go yet so there should be no downside to changing API now.
I think you should integrate these comments with the instructions at the top of the file.

Something like "Implement a circular buffer satisfying the io.ByteReader and io.ByteWriter interfaces. While the `ReadByte` method should check for overflow, `Overwrite` should be an unconditional write operation."? (Wording could be even better)