* optional: check registration of none type and optional type

* core: remove useless include

* core: fix include

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Co-authored-by: Justin Carpentier <justin.carpentier@inria.fr>

* optional: expose to-value converter for boost::none_t (and std::nullopt_t if possible)

+ add src/optional.cpp to expose none types
+ this allows to have None as a default arg value for functions (also, shows up in stubs)
+ also allows to have none_t return/argument type

* Run pre-commit