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

docs/source/yosys_internals/hashing.rst

@@ -36,7 +36,7 @@ The main characteristics are:
    all compilers, standard libraries and architectures.
 
 In addition to ``dict<K, T>`` and ``pool<T>`` there is also an ``idict<K>`` that
-creates a bijective map from ``K`` to the integers. For example:
+creates a bijective map from ``K`` to incrementing integers. For example:
 
 ::