incompatible change to Client.Rename function interface

[David Presotto]

On Monday, April 30, I will submit an incompatible change (https://upspin-review.googlesource.com/c/upspin/+/18940) to fix caching behavior of rename's in upspinfs.  upspin/upspin.go's Client.Rename's signature will change from:

Rename(oldName, newName PathName) error

to:

Rename(oldName, newName PathName) (*DirEntry, error)

In order to maintain a consistent cache, upspinfs needs to order both its own direct actions and WatchEvents coming from the server.  The ordering is accomplished using the Sequence

in the DirEntry, requiring the return or the DIrEntry.

A similar change was made to Put in the past and since Client.Rename performs a Put and a Delete, it too should have been changed.  I just missed it.