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_monitor.py

@@ -0,0 +1,22 @@
+from pyosys import libyosys as ys
+from pathlib import Path
+
+__file_dir__ = Path(__file__).absolute().parent
+
+d = ys.Design()
+
+class Monitor(ys.Monitor):
+	def __init__(self):
+		super().__init__()
+		self.mods = []
+
+	def notify_module_add(self, mod):
+		self.mods.append(mod.name.str())
+
+m = Monitor()
+d.monitors.add(m)
+
+ys.run_pass(f"read_verilog {__file_dir__ / 'spm.cut.v.gz'}", d)
+ys.run_pass("hierarchy -top spm", d)
+
+assert m.mods == ["\\spm"]