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.

1. Move common methods of AccountHeaders, ContainerHeaders,
   ObjectHeaders into a base type Headers.

2. Fold Headers into RequestOptions to remove one of the two optional
   arguments on request methods. The new Headers.ToOpts() method
   offers a nice experience for users passing Headers to request
   methods.

   The Update() methods keep the explicit Headers argument since the
   Headers argument is not optional there.

   The only downside is that we lose a bit of type-safety because
   RequestOptions takes any Headers instance, so e.g. ContainerHeaders
   could be passed to Object.Upload(). I believe the benefits outweigh
   this problem.

For use in Account.BulkDelete().