Fix crash in MakeImmutable(rec(x:=~));

[fingolfin]

Also adds some comments and cleans up the code (also for plists)

Fixes #1912

[ChrisJefferson]

Nice catch, and nice fix (I was worried we would have to store some kind of stack of visited objects).

While this does introduce a tiny window of problems in HPC-GAP, I think it will be OK for now. Longer term we could change to another TNUM (not sure which would be good, maybe need a new one), just to stop people touching the object while it is being changed.

[codecov]

Codecov Report

Merging #2601 into master will decrease coverage by 0.54%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2601      +/-   ##
==========================================
- Coverage   75.34%   74.79%   -0.55%     
==========================================
  Files         442      479      +37     
  Lines      228957   242240   +13283     
==========================================
+ Hits       172498   181195    +8697     
- Misses      56459    61045    +4586

Impacted Files	Coverage Δ
src/precord.c	95.12% <100%> (ø)	⬆️
src/plist.c	93.91% <100%> (ø)	⬆️
src/debug.c	36.84% <0%> (-36.85%)	⬇️
lib/session.g	56.86% <0%> (-33.77%)	⬇️
lib/random.g	93.33% <0%> (-6.67%)	⬇️
lib/info.gi	59.66% <0%> (-0.51%)	⬇️
lib/methwhy.g	50% <0%> (-0.22%)	⬇️
lib/profile.g	13.1% <0%> (-0.08%)	⬇️
lib/pager.gi	7.83% <0%> (-0.05%)	⬇️
hpcgap/lib/vec8bit.gi	49.73% <0%> (ø)
... and 60 more