Add BindingsOfClosure helper

[fingolfin]

Partially addresses #3569.

Well, or maybe it addresses it completely, though one still might want to enhance the output of Display(func) if there are bindings? Somebody else could do that based on this PR, though.

I also guess it would be nice to have documentation for BindingsOfClosure...

[codecov]

Codecov Report

Merging #3606 into master will decrease coverage by 0.9%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master    #3606      +/-   ##
==========================================
- Coverage   84.55%   83.65%   -0.91%     
==========================================
  Files         697      697              
  Lines      345018   345135     +117     
==========================================
- Hits       291723   288709    -3014     
- Misses      53295    56426    +3131

Impacted Files	Coverage Δ
lib/function.gi	86.45% <100%> (+2.5%)	⬆️
src/vars.c	93.85% <71.42%> (-0.21%)	⬇️
lib/grpreps.gi	37.36% <0%> (-46.71%)	⬇️
lib/ffeconway.gi	68.82% <0%> (-17.68%)	⬇️
lib/grpramat.gi	71.21% <0%> (-16.61%)	⬇️
lib/gprd.gi	59.15% <0%> (-15.44%)	⬇️
lib/polyconw.gi	65.24% <0%> (-14.19%)	⬇️
lib/grplatt.gi	64.4% <0%> (-11.83%)	⬇️
lib/helpview.gi	29.05% <0%> (-9.69%)	⬇️
lib/clashom.gi	71.17% <0%> (-9.24%)	⬇️
... and 219 more

[wilfwilson]

@DominikBernhardt and I have reviewed this together. Everything looks fine, and it's nice to have. Although we agree with you that it would be nice to have some documentation that gives the purpose and expected output of BindingsOfClosure.

[coveralls]

Coverage increased (+0.02%) to 84.323% when pulling c2d65b2 on fingolfin:mh/BindingsOfClosure into 0f7a8d6 on gap-system:master.

[wilfwilson]

@fingolfin Do you intend to document BindingsOfClosure in this PR?

[fingolfin]

I don't have the time and energy to work on documentation for BindingsOfClosure right now. I am not sure we'd want to document it as such globally, either... so there are multiple paths forward:

1. We merge this now, then for a few people BindingsOfClosure already is useful; and update Add a convenient way to display bindings inside of closures #3569 to mention it, suggesting that somebody else submit a PR to enhance Display for closures using it (and also that they could consider documenting BindingsOfClosure as well).
2. We merge it until somebody has time and energy to write some documentation, and tack it onto this PR.

I think 1. is better because it means this code won't bitrot, and people can benefit from it already (even if the number of people doing so would be small, as it is not documented). But either is fine by me.

[wilfwilson]

I'm happy for us to do 1. I thought that perhaps your sentence

I also guess it would be nice to have documentation for BindingsOfClosure...

suggested that you were maybe thinking of creating documentation, which is why I asked about it.