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_idict.py

@@ -0,0 +1,31 @@
+from pyosys import libyosys as ys
+
+my_idict = ys.IdstringIdict()
+print(my_idict(ys.IdString("\\hello")))
+print(my_idict(ys.IdString("\\world")))
+print(my_idict.get(ys.IdString("\\world")))
+try:
+	print(my_idict.get(ys.IdString("\\dummy")))
+except IndexError as e:
+	print(f"{repr(e)}")
+print(my_idict[0])
+print(my_idict[1])
+try:
+	print(my_idict[2])
+except IndexError as e:
+	print(f"{repr(e)}")
+
+for i in my_idict:
+	print(i)
+
+current_len = len(my_idict)
+assert current_len == 2, "copy"
+
+my_copy = my_idict.copy()
+my_copy(ys.IdString("\\copy"))
+assert len(my_idict) == current_len, "copy seemed to have mutate original idict"
+assert len(my_copy) == current_len + 1, "copy not behaving as expected"
+
+current_copy_len = len(my_copy)
+my_copy |= (ys.IdString(e) for e in ("\\the", "\\world")) # 1 new element
+assert len(my_copy) == current_copy_len + 1, "or operator returned unexpected result"