Remove all specific process functions that flatten data from arrays to strings

Problem/Motivation

Theme development for drupal is too complicated, with too many layers. We aim to remove the whole Process layer, but that can only be done in small pieces.

Proposed resolution

Since we can now flatten data structures to arrays to strings as part of the rendering process, we will need to go through all the process functions in core and remove this flattening. Examples of functions that can be removed include template_process().

Remaining tasks

- go through all process functions in core
- remove flattening everywhere it appears
- make sure flattening happens when content is rendered

API changes

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