Remove the need for remote.CheckPushPermission

This method employs a somewhat silly hack to determine reliably whether a keychain can be used to authorize pushes to a repo. This can be useful if you want to check whether you can push an image before you spend lots of effort constructing the layers for that image.

@jonjohnsonjr has some ideas about how we can make this unnecessary by allowing users to call `remote.Write` before all the layer contents are available and `mutate.Append`ed in, by passing `stream.Layer`s to it that are populated in goroutines and closed when they're done. This relies on a change to how `remote.Write`, `mutate.Append` and `stream.Layer` fit together.

I'll let him elaborate.