Structured symbolic arguments

[jberthold]

Implements generating symbolic arguments with all available (known) structure for custom start symbolis

TODO

• ncludes some draft code to be refactored (should use heating/cooling)
• A simple test should be added
• struct s are still abstracted completely because field type information is missing (stable-ir-json` change required)

[jberthold]

This code is now mostly obsolete because we are initialising symbolic variables in the python client.
Still leaving the branch intact in case we want to go back to it.