[toolbox] Move metafunctions required in core IO to core

[mloskot]

First stab at removing the confusing core IO dependency on toolbox and dynamic_image extensions, moving four metafunctions together with their tests: get_pixel_type, is_bit_aligned, is_homogeneous, pixel_bit_size.

Notice, the core IO dependency on the extensions still remains but it is not entirely clear: core IO uses any_image_view which seems to be pulled from dynamic_image indirectly through toolbox headers.

If IO relies on any_image and any_image_view, those should be moved to core as well.

Tasklist

• [ ] Review
• [ ] Adjust for comments
• [ ] All CI builds and checks have passed

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Confusing dependency

Regarding the existing confusion between the toolbox and the dynamic_image, the former seems to include the latter just to define this simple get_pixel_type:

https://github.com/boostorg/gil/blob/32fec9f05ba8d558c7af7b1bd537c80f31c2b826/include/boost/gil/extension/toolbox/metafunctions/get_pixel_type.hpp#L26-L30

pulls this which defines just a few tags

https://github.com/boostorg/gil/blob/2250b7159c398899ec0fa27246f0d7aacdf2c1fe/include/boost/gil/extension/toolbox/dynamic_images.hpp#L18-L27

then pulls #include <boost/gil/extension/dynamic_image/dynamic_image_all.hpp> to get the whole dynamic_image extension.

Plan proposal

1. Merge this PR
2. In current gil/*.hpp and gil/io/*.hpp core headers, find what else is required from toolbox and dynamic_image
3. Move the required parts to the core headers.

It means, the move direction should be from extensions to core, not the other way around, which would be much harder task, I think.

[mloskot]

*** MARKING AS WIP ***

On reflection, I'm postponing this until the whole clang-tidy refactoring is done. Moving files around while changing them may not be a good idea, esp. if we predict that is not all moving of files we may have to do :)