python: add enum binder that adds missing ops to enum classes
python: add enum binder that adds missing ops to enum classes

pybind11 bundled `enum_` binder adds binary ops to Python enum only if
C++ enum implicitly convertible to underlying type. This condition is
always false for `enum class`.

Provide additional BS binders for enums that adds missing operators and
additionally bind `+` and `-` ops (can be useful for bitwise flags).

uentity/blue-sky-re,uentity/blue-sky-re,uentity/blue-sky-re