Investigate reducing MAXVERTS in 32-bit mode

[wilfwilson]

@ChrisJefferson suggested this as a possibly good idea in https://github.com/semigroups/Semigroups/issues/634#issuecomment-790667663. Memory is severely limited on 32-bit mode, and with MAXVERTS = 512, Digraphs can apparently end up being quite demanding on the mrmory. Reducing MAXVERTS should theoretically make Digraphs more stable on 32-bit systems, at the cost of it, unfortunately, being less useful.

Alternatively, or in combination, we might want to be cleverer about how we allocate memory (especially for stabiliser chains); see #433.

[james-d-mitchell]

Just to mention #602: we should close this issue if we drop 32-bit support.