Replace boost::fusion with iris::alloy

[yaito3014]

TODO

• Add natvis

Post-merge TODO

• Remove Boost.MPL dependency
• Remove Boost.Core dependency
• Limit Boost.PP dependency to development build only
• Refine X4Attribute to include the default constructible / assignable trait

[saki7]

So far so good.

The CI workflow must be changed like below:

• If "general" files are modified, set a flag to test both alloy and x4
• If "alloy" files are modified, set a flag to test both alloy and x4
• If "x4" files are modified, set a flag to test only x4

In other words the "general", "alloy", and "x4" changes must be used for determining whether to build alloy/x4. The last workflow run shows that they're treated like some standalone target, which resulted in building all libraries in duplicated runs.

When the workflow is corrected, it must only show "alloy" and/or "x4" category in the workflow run. No "general" category should be run, because there's no targets placed directly under the test/ directory.