Feature/flatten serialization

[kartik4949]

Description

Related Issues

Checklist

• Is this code covered by new or existing unit tests or integration tests?
• Did you run make unit-testing and make integration-testing successfully?
• Do new classes, functions, methods and parameters all have docstrings?
• Were existing docstrings updated, if necessary?
• Was external documentation updated, if necessary?

Additional Notes or Comments

[blythed]

Solution for inline-encoding

If a Document has a Schema (e.g. in ibis all Document instances have a Schema) then:

• bytes are encoded directly (don't need a _BaseEncodable in "_leaves".
• if Encodable: then the bytes are saved directly in the field (as raw or Base64)
• if Artifact: then the bytes are referred to with &<file_id>
• if File: then the file is referred to with file://

[jieguangzhou]

I rebased the code and created a public branch for collaborative commits.
#2056