Updates to the Julia GC integration

[rbehrends]

This brings the implementation in line with the most recent API changes for the Julia GC extensions and cleans up some code.

[codecov]

Codecov Report

Merging #2688 into master will increase coverage by 0.12%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           master    #2688      +/-   ##
==========================================
+ Coverage   75.75%   75.87%   +0.12%     
==========================================
  Files         478      481       +3     
  Lines      241495   241314     -181     
==========================================
+ Hits       182940   183094     +154     
+ Misses      58555    58220     -335

Impacted Files	Coverage Δ
src/gasman.h	95.12% <ø> (+0.25%)	⬆️
src/gap.c	84.42% <0%> (-0.83%)	⬇️
src/weakptr.c	97.72% <100%> (-0.01%)	⬇️
lib/init.g	71.35% <40%> (-0.42%)	⬇️
src/io.c	59.9% <0%> (-1.13%)	⬇️
src/calls.c	93.58% <0%> (-1.07%)	⬇️
src/objcftl.c	23.46% <0%> (-0.85%)	⬇️
hpcgap/lib/hpc/stdtasks.g	63.42% <0%> (-0.77%)	⬇️
lib/oper.g	80.96% <0%> (-0.39%)	⬇️
... and 64 more

[fingolfin]

Since those Julia changes are not yet quite stable, let's wait with merging this for now.

For the status of the Julia side of things, see here: JuliaLang/julia#28368

[fingolfin]

Memo to @rbehrends and myself: we should also modify init.g to print something like with Julia GC or even with Julia 1.0 or so (and then export from the kernel JULIA_VERSION, just like GMP_VERSION is exported).

[fingolfin]

These changes seem likely to be ready for the "final" Julia GC extension interface (although that still hasn't been merged). For user convenience, it'd be nice to still get this into GAP 4.10. Since the code in here is disabled for non-Julia users, that should cause no issues.