Update ORC statistics API to use C++17 standard library

[vuule]

Uses std::optional instead of std::unique_ptr for optional ORC statistics fields, and std::variant instead of the manually implemented type erasure.
Removes a lot of code that simulated std::optional and std::variant when it wasn't available.

[vuule]

Breaking change because it modifies the type that read_parsed_orc_statistics returns.

[vuule]

Rerun tests

[codecov]

Codecov Report

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  Files                   ?      105           
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  Branches                ?        0           
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  Hits                    ?    16874           
  Misses                  ?     3102           
  Partials                ?        0           

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[hyperbolic2346]

I really like how so much of this melted away.

[vuule]

I really like how so much of this melted away.

Yeah, I wrote so much code to approximate std::variant and std::optional functionality. So glad to be able to remove it :)

[trxcllnt]

👍

[nvdbaranec]

Nice work.

[vuule]

rerun tests

[vuule]

@gpucibot merge