Add `deep_size` field to manifest

[holisticode]

When accessing a directory, a web-page or dapp, it is currently not possible to evaluate the file of the complete download.

For incentivization, this could pose several problems. Read for example #1438 and #1437

This issue proposes to evaluate if adding a deep_size field to a manifest makes sense and is feasible. When uploading the complete tree of files, we should be able to know the complete size of the upload of all components; thus, we should be able to populate such a field.

@acud @nonsense @zelig