Add GetWithDefault

[ChrisJefferson]

GetWithDefault provides a way of checking if an index in a list
is bound, and the value if the index is bound, as one function call.

It is particularly useful in weak pointer lists and atomic lists,
where there is a race condition between checking if an element is
bound, and then reading it's value.

This removes the very recently added method CHECKED_GET_ATOMIC_LIST, as it is now subsumed into this general method.

[codecov]

Codecov Report

Merging #1762 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1762      +/-   ##
==========================================
+ Coverage   60.92%   60.93%   +<.01%     
==========================================
  Files         997      997              
  Lines      301950   301970      +20     
  Branches    13036    13040       +4     
==========================================
+ Hits       183968   184007      +39     
+ Misses     115333   115320      -13     
+ Partials     2649     2643       -6

Impacted Files	Coverage Δ
lib/hpc/thread1.g	68.18% <ø> (-1.82%)	⬇️
src/hpc/aobjects.c	62.79% <100%> (+0.34%)	⬆️
src/weakptr.c	82.67% <100%> (+0.41%)	⬆️
src/lists.c	56.06% <100%> (+0.97%)	⬆️
src/lists.h	100% <100%> (ø)	⬆️
src/hpc/thread.c	46.64% <0%> (-0.2%)	⬇️
src/funcs.c	71.82% <0%> (ø)	⬆️
src/stats.c	73.97% <0%> (+0.27%)	⬆️
src/hpc/threadapi.c	34.74% <0%> (+0.28%)	⬆️
src/hpc/traverse.c	78.21% <0%> (+0.3%)	⬆️
... and 2 more

[sebasguts]

Thanks, this will save a lot of if's when using weak pointers.

I would suggest changing the manual entry for retrieving an element from a weak pointer list to use this command as suggested way to get elements from the weak pointer list.