Add IsAutoGlobal for testing whether a variable was declared using DeclareAutoreadableVariables

[ChrisJefferson]

At the moment there is no way to find if a variable is an "auto" (which means a function will be run when you access it). This is a problem in the (rare) cases where one wants to iterate over every variable in GAP, as just reading the value of an auto variable can cause lots of code to run.

Fixes (hopefully) a bug in the SYNTAX_TREE test

[fingolfin]

I do realize you just copied the style used elsewhere in this file, but this inconsistent with what we do elsewhere. The following three alternatives come to mind (and we sadly use all of them in some place or another; perhaps we ought to come up with some style guidelines for our docs, and try to enforce them globally?)

Suggested change

	## IsAutoGlobal ( <A>name</A> ) returns true if there is a global variable
	## returns true if there is a global variable

or

Suggested change

	## IsAutoGlobal ( <A>name</A> ) returns true if there is a global variable
	## Returns true if there is a global variable

or

Suggested change

	## IsAutoGlobal ( <A>name</A> ) returns true if there is a global variable
	## <Ref Func="IsAutoGlobal"/> returns true if there is a global variable

[ChrisJefferson]

I'm not opposed to these changes, but I would also prefer if we did try to keep the various *Global functions consistent with each other, so change them all, or none of them.

[fingolfin]

OK, then let's do those with another PR someday in the future.

[codecov]

Codecov Report

Merging #3076 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3076      +/-   ##
==========================================
+ Coverage   83.85%   83.85%   +<.01%     
==========================================
  Files         688      688              
  Lines      343238   343230       -8     
==========================================
+ Hits       287818   287822       +4     
+ Misses      55420    55408      -12

Impacted Files	Coverage Δ
lib/global.gi	53.67% <100%> (+1.9%)	⬆️
src/gvars.c	86.15% <100%> (+0.47%)	⬆️
lib/global.gd	100% <100%> (ø)	⬆️
lib/record.gi	75.94% <0%> (-2.63%)	⬇️
lib/files.gd	74.28% <0%> (-0.37%)	⬇️
lib/record.gd	100% <0%> (ø)	⬆️
lib/string.gd	100% <0%> (ø)	⬆️
lib/string.gi	77.88% <0%> (+0.43%)	⬆️
lib/files.gi	68.04% <0%> (+1.92%)	⬆️

[ChrisJefferson]

I've fixed the bug and added a test for hpcgap (thanks @fingolfin). I'd prefer to not change the docs here, but make larger consistent changes in another PR.