fixed cross join serialization

[fnc12]

• fixed cross_join serialization and fixed cross_join.cpp example
• moved cross_join_t to a dedicated file in ast folder
• added is_cross_join trait
• removed cross join and natural join string superclasses and moved all serialization logic to statement_serializer

@trueqbit probably I messed up with polyfill API - please feel free to add comments

[trueqbit]

You are doing great with using polyfills :)

[cursor]

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[fnc12]

this Cursor ambush was unexpected and happened due to my GitHub connection with Cursor account. I did not explicitly asked Cursor to make reviews here, it is a result of their shitty marketing. I disabled connection with GitHub to prevent this in future. Sorry for that