Add RequireMutable

[ChrisJefferson]

This adds the C function "RequireMutable".

This took more work than I planned, as I found errors of the type:

" must be mutable (not a ...)" a bit confusing, as for this error I found the "not a ..." bits misleading.

[coveralls]

Coverage increased (+0.004%) to 85.158% when pulling 3cdf2c8 on ChrisJefferson:require_mutable into 6f1661e on gap-system:master.

[wilfwilson]

I like this PR and the fact that it simplifies the code.

I asked one question about the removed ErrorReturnVoid, I just want to make sure it was a deliberate decision to change that behaviour, if so I’m happy to approve.

My only complaint is that the error messages have less information, but then again the error that is given corresponds exactly to the condition that is being tested. So fair enough.

[fingolfin]

Generally a good thing! But some remarks (inspired by @wilfwilson )

[wilfwilson]

Given that @fingolfin explained my question to me, I approve the PR; if you want to add more information into the error messages, as discussed in the thread, then that would be welcome, but I don't insist on it.

[codecov]

Codecov Report

Merging #3391 into master will increase coverage by 0.35%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #3391      +/-   ##
==========================================
+ Coverage   85.15%    85.5%   +0.35%     
==========================================
  Files         697      646      -51     
  Lines      344073   317133   -26940     
==========================================
- Hits       292998   271168   -21830     
+ Misses      51075    45965    -5110

Impacted Files	Coverage Δ
src/error.h	88.88% <ø> (-11.12%)	⬇️
src/lists.c	75% <100%> (-0.17%)	⬇️
src/listfunc.c	93.47% <100%> (-0.61%)	⬇️
src/vecgf2.c	72.6% <62.5%> (-1.12%)	⬇️
lib/random.g	66.66% <0%> (-30.56%)	⬇️
lib/read2.g	85.71% <0%> (-14.29%)	⬇️
lib/read1.g	85.36% <0%> (-13.42%)	⬇️
lib/session.g	57.69% <0%> (-12.83%)	⬇️
src/calls.h	87.59% <0%> (-10.29%)	⬇️
... and 173 more

[ChrisJefferson]

This patch got massively simplified :) But I think the messages are much better. Thanks for all feedback.

I did keep the patch to remove the extra message, but I feel once we say "must be a mutable list", it's fine to then say "(not a ...)"

[fingolfin]

Nice! One minor remark, but merging as-is would be fine as well!