Interface for writing exported API

[jonasfj]

This is a proposal for an interface for writing the exported API.

The idea is to put everything related to managing the actual objects of the bucket into a single abstraction.

This interface isn't responsible for figuring out:

• what to write, or,
• when to write it.

Only where to write things.

The hope is that this makes it less likely that we accidentally writes file to the wrong location, forget to update them, or forget to GC particular files.