pyosys: rewrite using pybind11

- Rewrite all Python features to use the pybind11 library instead of boost::python.
  Unlike boost::python, pybind11 is a header-only library that is just included by Pyosys code, saving a lot of compile time on wheels.
- Factor out as much "translation" code from the generator into proper C++ files
- Fix running the embedded interpreter not supporting "from pyosys import libyosys as ys" like wheels
- Move Python-related elements to `pyosys` directory at the root of the repo
- Slight shift in bridging semantics:
  - Containers are declared as "opaque types" and are passed by reference to Python - many methods have been implemented to make them feel right at home without the overhead/ambiguity of copying to Python and then copying back after mutation
  - Monitor/Pass use "trampoline" pattern to support virual methods overridable in Python: virtual methods no longer require `py_` prefix
- Create really short test set for pyosys that just exercises basic functionality

kernel/yosys_common.h

@@ -55,6 +55,9 @@
 
 #ifdef WITH_PYTHON
 #include <Python.h>
+#include <pybind11/pybind11.h>
+
+namespace py = pybind11;
 #endif
 
 #ifndef _YOSYS_