The additional guarantee that Object.SymlinkHeaders becomes equivalent
to Object.Headers for non-symlinks will be useful e.g. for
swift-http-import's usecase.

This has gone through a lot of iterations on my branch, and I'm quite
happy with the parts of the API that exist now. Test coverage can still
be better, and will get better in the following commits.

The API is not yet finished: I want to add Options arguments to
Object.Upload(), Object.Copy(), Object.Move() and Object.Delete() that
specify how each of these operations affect existing segments (and,
later, also existing symlinks). For Upload(), uploading in segments
shall become as easy as flipping a single switch.