A huge amount of this is repetitive:
- `boost::get<T>(variant)` becomes `std::get<T>(variant)`
- `boost::get<T>(variant_ptr)` becomes `std::get_if<T>(variant_ptr)`
- `variant.type() == typeid(T)` becomes `std::holds_alternative<T>(variant)`
There are also some simplifications to visitors using simpler stl
visitors, or (simpler still) generic lambdas as visitors.
Also adds boost serialization serializers for std::variant and
std::optional.