Reverse the parameters of Retry

[bobg]

The Operation argument to Retry is likely to be a (possibly lengthy) inline function literal. This can make it hard to locate the BackOff argument that must follow it. It would be slightly more ergonomic to be able to write:

err = backoff.Retry(bkoff, func() error {
  ...
})

[cenkalti]

Good idea. This can be implemented in v5.