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.

Everything in one package once more. The bigger API in *this* package is
worth it when we don't have to expose guts to cross package boundaries.

This allows me to make the API much simpler. More simplification
forthcoming in the following commit; I just want to make a cut since
`make test` is happy right now.

I'm quite satisfied with this right now (though this doesn't say
anything about how I feel about it tomorrow), but it's ugly that some
guts (headers.Base) are exposed in the public API.