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

.github/workflows/wheels/cibw_before_build.sh

@@ -11,24 +11,3 @@ if [ "$(uname)" != "Linux" ]; then
 fi
 python3 --version
 python3-config --includes
-
-# Build boost
-cd ./boost
-## Delete the artefacts from previous builds (if any)
-rm -rf ./pfx
-## Bootstrap bjam
-./bootstrap.sh --prefix=./pfx
-## Build Boost against current version of Python, only for
-## static linkage (Boost is statically linked because system boost packages
-## wildly vary in versions, including the libboost_python3 version)
-./b2\
-    -j$(getconf _NPROCESSORS_ONLN 2>/dev/null || sysctl -n hw.ncpu)\
-    --prefix=./pfx\
-    --with-filesystem\
-    --with-system\
-    --with-python\
-    cxxflags="$(python3-config --includes) -std=c++17 -fPIC"\
-    cflags="$(python3-config --includes) -fPIC"\
-    link=static\
-    variant=release\
-    install