Add support for assigning references in importer API

[amykyta3]

Currently the importer API only allows building components with "regular" property values. Add support for assigning instance and property references to component properties.

Some considerations to be aware of:

• References that involve array indexes may require more complex construction, since simply providing the terminal endpoints is not enough information.
  • Need an intuitive way for the user to be able to provide the lineage of array indexes as well.
  • Alternatively, do not support this yet?
• ~~Will likely need to add a parent member to the Component class to facilitate construction of the internal ComponentRef object. I believe in most/all cases, parent shall never be deepcopied (copy by ref instead).~~
  • No longer possible to use this method. Back-end optimizations in 1.30.0 likely prevent this since identical Component instances get re-used, so they cannot have a common parent attribute.
• This will likely be implemented as Importer.create_inst_reference() and Importer.create_prop_reference() methods.
  • Inputs are the source component, destination component, and source property name (for prop refs)
  • Returns a ComponentRef/PropertyRef object respectively.
• When assigning the reference object via Importer.assign_property(), check that a DPA is allowed.
  • Since prop refs imply a DPA, check that spec rules are not being violated inadvertently.

[Risto97]

Hello @amykyta3. This issue is blocking me right now. I want to be able to assign a field instance to next, enable, field properties.

I know you are probably busy to implement it. If you already started working on it, or could give some more pointers, I could try to implement it.

I see for now that the assign_property() does not allow value to be a Component https://github.com/SystemRDL/systemrdl-compiler/blob/191f4dd9bc093920720ddd47e8a15be97e6c5a25/systemrdl/importer.py#L312-L314