pyosys: fix a number of regressions from 0.58

[donn]

What are the reasons/motivation for this change?

• An overwhelming number of dangling reference-related issues
• AttrObject methods were not available for Cell

Explain how this is achieved.

• all class members, including pointers and containers, are now copied by value
  • pybind11 does a decent job bridging them appropriately-- the only surprising element to this is that a.b.append(x) mutates a copy of a.b. but this was the previous behavior.
• add new overload of RTLIL::Module::addMemory that does not require a "donor" object
  • the idea is Module, Memory, Wire, Cell and Process cannot be directly constructed in Python and can only be added to the existing memory hierarchy in Design using the add methods - Memory requiring a donor object was the odd one out here
• fix superclass member wrapping only looking at direct superclass for inheritance instead of recursively checking superclasses
• fix superclass member wrapping not using superclass's denylists
• fix Design's __str__ function not returning a string
• fix the generator crashing if there's any std::function in a header
• misc: add a crude __repr__ based on __str__

Make sure your change comes with tests. If not possible, share how a reviewer might evaluate it.

With a Yosys compiled with ENABLE_PYTHON, python3 ./tests/pyosys/run_tests.py yosys

[mmicko]

Would like to ping @jix and @georgerennie on this one as well

[donn]

I should mention for the record there's another lifecycle issue left which is dangling references behave a lot like C++. The way the boost bindings handled that is by keeping the entire design in memory if any references to even like, an IdString key in cells_ was left. I will replicate that behavior in another PR, but this fixes a more immediate issue.

[donn]

Actually, let me think about this one a bit more. The copy by value solution is also kind of a hack — because if A and B are classes and A has a non-pointer member of type B, a.b.x = 0 would mutate a copy. This will confuse people.

The current semantics are consistent despite the memory issues. I just need to fix the memory issues.