Core: C++ improvents

[iopapamanoglou]

Feature Request

• make everything const
• fix leakage
  • fix cyclic shared ptr
• be careful with reusing G
  • just use Module everywhere
• revisit factories
  • maybe better to just override init to return shared_ptr?
  • see also nb::cast

Code of Conduct

• I agree to follow this project's Code of Conduct