32-bit support

[james-d-mitchell]

My reading of @fingolfin's last comment on:

https://github.com/gap-actions/setup-gap/issues/45

is that 32-bit continuous integration for GAP is not currently feasible (or at least not worth the effort). I tend to agree, I don't think this is the same as dropping support for 32-bit though, I'd suggest we retain 32-bit support until either:

• it is found to be hopelessly broken, or
• GAP stops supporting 32-bit officially,

At that point we can drop support for 32-bit, and remove any relevant special code. What do @mtorpey and @wilfwilson say?

Originally posted by @james-d-mitchell in https://github.com/digraphs/Digraphs/issues/587#issuecomment-1884729038

[mtorpey]

After discussion in person:

• We will leave the 32-bit functionality and tests open for now;
• If the 32-bit tests start failing, we'll disable them;
• If our 32-bit support is found to be hopelessly broken, or GAP stops supporting 32-bit officially, we'll remove everything.