RAII, smart pointers etc.

[egorpugin]

After we upgraded to C++17, did cleanups on STRINGs and GenericVectors, it's time to use more RAII, more smart pointers.

Modernization must be started from unit tests and training tools. Because if we change something in core library (from the bottom), we must clean all the mess above (tess.exe, tr.tools and tests). Starting from the top is much easier.

Note that this affects only internals. We do not touch public APIs.

cc: @stweil

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Upgrade tips:

• replace new with std::make_unique
• replace returned pointers that must be deleted later with unique_ptrs (auto deletion)
• use more auto (AAA - almost always auto): auto v = something;
• use more std::string or std::string_view (to be determined what to use and where)
• replace pointer parameters with references where null values is not intended
• leptonica raii too

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Some new places: unittests: https://pastebin.com/ki8Fpt3C (partially fixed) training tools: https://pastebin.com/puKvFT7M (partially fixed)

[amitdo]

@egorpugin added the tess6.0 label

Egor, maybe you want to use a milestone instead of a label?

[egorpugin]

ok, ty :)

[egorpugin]

For the info: this ticket is blocked by api changes. See https://github.com/tesseract-ocr/tesseract/commit/7d17b72ba5d10bbd0c65a1992ddba8ec6ac1dcd0#r48512779

[egorpugin]

For leptonica raii see https://github.com/tesseract-ocr/tesseract/pull/921