julia_gc: include julia.h globally for better CHANGED_BAG

[fingolfin]

CHANGED_BAG is supposed to have minimal overhead, which normally is achieved by inlining it. But we never had this for the Julia GC, mainly because it was painful to do so in our C++ source files. This PR addresses this.

As a side effect, this enables future work I have planned that will need the Julia headers globally.

Note that CHANGED_BAG cannot simply call jl_gc_wb_back: Julia declares that as static inline, and C forbids referencing an identifier with internal linkage from an inline function with external linkage. Marking CHANGED_BAG as static inline instead is not an option either, as it is used by other EXPORT_INLINE functions (such as PushPlist), which then run into the very same problem. So we inline a copy of Julia's write barrier, which only refers to jl_gc_queue_root.

Since src/gasman.h now includes julia.h, all GAP kernel extensions need the Julia headers, too, so we pass those on to packages via sysinfo.gap. The JULIA=yes CI job now also runs testmockpkg to catch regressions here.

This also means that macros defined by julia.h can now clash with code in kernel extensions. One such clash affects GAP.jl's JuliaInterface, which declares jl_n_threads itself; the CI with GAP.jl jobs will therefore keep failing until oscar-system/GAP.jl#1412 has been merged.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.03%. Comparing base (c0ec89a) to head (674a54f).

Files with missing lines	Patch %	Lines
src/gasman.h	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6490   +/-   ##
=======================================
  Coverage   79.03%   79.03%           
=======================================
  Files         685      685           
  Lines      293790   293793    +3     
  Branches     8664     8642   -22     
=======================================
+ Hits       232203   232206    +3     
  Misses      59786    59786           
  Partials     1801     1801           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[fingolfin]

This may need some more refining...