Implement modules support

[yesmanchyk]

This implementation is based on bemanproject/exemplar#380

There are some key differences, however:

1. I use import beman.scan_view; instead of #include <beman/scan_view/scan.hpp> in tests and examples.
2. I had to include all dependencies library feature-test macros into global module fragment because import std; resulted in compilation errors when I tried gcc 16 and clang 21.
3. Due to above 2 reasons I extracted the beman::scan_view implementation into scan_view.hpp without any changes.

The cmake --build build step takes longer when configured with -DBEMAN_SCAN_VIEW_USE_MODULES=On - here is what Jason Merrill from Red Hat replied about slowdown in gcc 16.1:
I imagine it takes longer because it has to build module std, which includes the whole standard library, before building anything else. I don't know if there's a simple way to teach cmake to share std.gcm between projects/builds.

[ednolan]

Putting standard library includes in the global module fragment is not the approved modules mechanism for this project. To merge this, you'll need to figure out how to get import std building. The best approach would be to do so by building with a supported container or with beman-local-ci.

[yesmanchyk]

I solved it in yesmanchyk@f196c4b. However, the line 7 looks like a hack to me even though CI is passing. Maybe it's worth restoring the global module fragment with a single #include <version> inside? I don't know a way to export library feature-test macros with import std;.

[Mick235711]

One way is import <version>, but I'm not sure header units are an approved mechanism, nor am I sure all targets support header units well enough...

As mentioned below, it seems that #include <version> (and things like <cstdarg>, <cassert>) is now allowed as a special exception in the GMF.

[yesmanchyk]

import <version>; fails compilation with the following:

error: failed to read compiled module: No such file or directory
note: compiled module file is 
'gcm.cache/./usr/lib/gcc/aarch64-unknown-linux-gnu/16/include/g++-v16/version.gcm'
note: imports must be built before being imported

plus I'm not sure macros will be visible that way

[ednolan]

Great work! Just a few more small comments. By the way, you can use pre-commit run --all-files as a convenient way to apply clang-tidy and ensure that the lint check passes.

[Mick235711]

Thanks a lot for working on this!

[yesmanchyk]

Looks like I've addressed all comments - please merge if it looks good.

[ednolan]

Great job!