LibGAP API: Add GAP_AssignGlobalVariable and GAP_CanAssignGlobalVariable

[sebasguts]

Description

Adds GAP_AssignGlobalVariable and GAP_IsNameOfWritableGlobalVariable to the libgap API, to assign a global variable to a value and check if a name belongs to a writable gvar.

Text for release notes

LibGAP API: GAP_AssignGlobalVariable and GAP_IsNameOfWritableGlobalVariable added

Further details

If necessary, please provide further details here.

Checklist for pull request reviewers

• proper formatting

If your code contains kernel C code, run clang-format on it; the
simplest way is to use git clang-format, e.g. like this (don't
forget to commit the resulting changes):

git clang-format $(git merge-base master)

• usage of relevant labels

  1. either release notes: not needed or release notes: to be added
  2. at least one of the labels bug or enhancement or new feature
  3. for changes meant to be backported to stable-4.X add the backport-to-4.X label
  4. consider adding any of the labels build system, documentation, kernel, library, tests

• runnable tests

• adequate pull request title

• well formulated text for release notes

• relevant documentation updates

• sensible comments in the code

[coveralls]

Coverage decreased (-0.001%) to 85.152% when pulling 61fc5b0 on sebasguts:sg/libgap-gvars into 84cd55e on gap-system:master.

[codecov]

Codecov Report

Merging #3438 into master will decrease coverage by 0.26%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3438      +/-   ##
==========================================
- Coverage   85.34%   85.08%   -0.27%     
==========================================
  Files         699      698       -1     
  Lines      346092   346096       +4     
==========================================
- Hits       295374   294468     -906     
- Misses      50718    51628     +910

Impacted Files	Coverage Δ
src/libgap-api.c	1.16% <0%> (-62.1%)	⬇️
src/compiler.c	66.99% <0%> (-21.71%)	⬇️
src/vars.c	81.58% <0%> (-12.49%)	⬇️
src/error.c	77.56% <0%> (-10.65%)	⬇️
src/stats.c	85.94% <0%> (-9.24%)	⬇️
src/exprs.h	93.75% <0%> (-6.25%)	⬇️
src/exprs.c	93.16% <0%> (-4.75%)	⬇️
src/intrprtr.c	95.3% <0%> (-4.54%)	⬇️
src/funcs.c	96.36% <0%> (-2.43%)	⬇️
... and 12 more

[sebasguts]

Thank you for updating and merging :)