ENH: dependencies, first pass at serialization, and more

[klauer]

• Added dependency store, largely borrowed from an EPICS-focused side project that has an optional blark dependency (whatrecord)
  • Parse project + all dependencies that have source code available
  • Using the summary classes, relatively easily poke around with source code objects
• Removed unused dataclasses + grammar rules
  • Closes Remove unused grammar rules #18
  • Up for discussion if I'm missing something
  • Task config, instruction lists, SFC networks, ...
• Grammar fixes
  • Functions can return indirect types
  • Added variable attribute flags to blocks where accepted, according to infosys docs
• Improved code summary utilities
  • Easier to use to find declarations and such
  • Add ProgramSummary, GlobalVariableSummary
  • Base type EXTENDS declarations are wrapped into the derived type
• Added program and global variables to documentation generation (sphinx domain)
• Added apischema-based serialization in the test suite
  • This helped improve the type annotations in places as round-tripping validation was failing
  • However, Python 3.9+ with apischema 0.17.5 (did not test other versions) is hitting an infinite loop when attempting to serialize most of the data structures. Serialization testing for those Python versions is disabled for now until I get a chance to dig in / report the issue if it's not on my end
  • Python 3.7 and Python 3.8 are testing serialization with the transformed dataclasses just fine though