[FLATTEN-ENCODING] Make encoding of `Document` less nested and easier to write

[blythed]

As a user specifying a Stack or Component using the Document formalism, I don't want deep nesting, rather shallow nesting, so that it's possible to easily write these specifications in yaml without too much trouble and mental gymnastics.

Specification:

_leaves
  - identifier: 123231ae1f212567896343444
    cls: Artifact
    module: superduperdb.components.datatype
    dict:
      datatype: pil_image

x: $_leaves[-1]

This would give a decoding as a Document.unpack() of {'x': <PIL.Image.PNGImagePlugin 1234x3>}. This can be applied to all objects which require encoding.