Fix C++20 global module fragment inclusion

[DonutVikingChap]

Using the experimental C++20 modules support in recent builds of Clang, EnTT currently causes compilation errors when included as part of the global module fragment in a module interface unit that exports part of EnTT's API and is then imported and used by a different translation unit. Here is a minimal example (note the "error: no matching function for call to 'popcount'" in the output): https://godbolt.org/z/9vTjz9Prh

The reason seems to be that certain function declarations that are implicitly required by the implementation aren't exported into the module interface since they are erroneously marked 'static' and have internal linkage when they should (presumably) be 'inline'. I found that changing these declarations accordingly fixed the problem and made EnTT work well with C++20 modules in my project, and this PR reflects those changes.

Unfortunately I can't write proper test cases for this since it depends on features that are still experimental in most compilers, so there may still be other static declarations that I have missed that could cause problems in other modules-based projects, but as long as these changes don't break anything for regular non-modules builds, they should hopefully at least help make EnTT a bit more modules-friendly in the short term.

[skypjack]

Makes sense actually, thanks for pointing this out. Why inline though? We can just drop the static and that's it.
Those are template functions, no need to make them inline to avoid symbols duplication. On the other hand, I'm pretty sure that the compiler knows better than me whether to inline them or not.
So, yeah, I'd rather get rid of it if possible before merging. Thanks. 👍

[codecov-commenter]

Codecov Report

Merging #1047 (1aa76da) into wip (ed64c28) will not change coverage.
Report is 3 commits behind head on wip.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##              wip    #1047   +/-   ##
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  Coverage   99.98%   99.98%           
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  Files         142      142           
  Lines       24440    24431    -9     
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- Hits        24436    24427    -9     
  Misses          4        4           

Files Changed	Coverage Δ
src/entt/entity/entity.hpp	100.00% <ø> (ø)
src/entt/core/type_info.hpp	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[DonutVikingChap]

Ok, the inline specifiers are removed now. In my own projects I usually mark all header-defined free functions either inline or constexpr even when they're templates. It's mostly for consistency, but also to make sure that the inline specifier is not accidentally forgotten in case the function is later refactored to not be a template, though I agree that it's not strictly necessary here.

[skypjack]

Yeah, don't get me wrong. What you say makes perfectly sense. I'm just trying to stick with the coding style of this project. 👍
Thanks for changing it. Let me know if you spot any other changes that are useful to make it work with C++20.
I also want to create a C++20 branch of the library soon, most likely after the vacations. 🥳