Create a pre-compile step based on clang-AST library to synthesize the type-erasing wrapper generation

[jbcoe]

C++ reflection is currently unable to generate one class definition from another. We'll add a pre-compile step that uses a C++ AST to emulate what we'd like to see standardized.

https://github.com/FFIG/ffig/blob/master/ffig/cppmodel.py

• Test and complete C++ code model from https://github.com/jbcoe/py_cppmodel
• Add pre-compile step to synthesize type-erasing class wrapper
• Add CMake support for pre-compile step
• Add Bazel support for pre-compile step
• Determine what new features need adding for C++ 29 reflection