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

tests/pyosys/test_dict.py

@@ -0,0 +1,13 @@
+from pyosys import libyosys as ys
+
+my_dict = ys.StringToStringDict()
+my_dict["foo"] = "bar"
+my_dict.update([("first", "second")])
+my_dict.update({"key": "value"})
+for key, value in my_dict.items():
+    print(key, value)
+
+new_dict = my_dict | {"tomato": "tomato"}
+del new_dict["foo"]
+assert set(my_dict.keys()) == {"first", "key", "foo"}
+assert set(new_dict.keys()) == {"first", "key", "tomato"}