Track kernel function definition locations more precisely

[fingolfin]

Resolve C definition lines lazily for standard GVAR-backed kernel functions. The result is cached, and used for StartlineFunc and PageSource.

Before:

gap> LocationFunc(RETURN_FIRST);
"src/gap.c:RETURN_FIRST"

After:

gap> LocationFunc(RETURN_FIRST);
"GAPROOT/src/gap.c:448"

This then allows us to drop the now unused LOCATION_BODY accessors, rename the shared body field to startline, and remove the leftover empty-string initialization for undefined global functions.

AI-assisted: OpenAI Codex was used to implement the change and prepare the regression coverage.

---

I actually wrote this back in April but then didn't want to bother spending time to get this merged.

[codecov]

Codecov Report

❌ Patch coverage is 72.82609% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.08%. Comparing base (74a43d6) to head (f30b6bd).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/calls.c	73.25%	23 Missing and 23 partials ⚠️
src/modules.c	66.66%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6427      +/-   ##
==========================================
- Coverage   78.86%   76.08%   -2.79%     
==========================================
  Files         685      642      -43     
  Lines      293551   278443   -15108     
  Branches     8672     7858     -814     
==========================================
- Hits       231519   211848   -19671     
- Misses      60225    65020    +4795     
+ Partials     1807     1575     -232     

☔ 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.

[lgoettgens]

CI unfortunately fails. I can have a look once this is green