Add iterators, Whitney numbers, characteristic polynomial, etc

[gmou3]

Changes (mostly in matroid.pyx):

• Add _iterator functions for bases, circuits, etc, and use them internally (this avoids memory consumption/blowups)
• Add Whitney numbers functions (for the first and second kind)
• Correct f_vector definition (the definition up till now referred to the Whitney numbers of second kind)
• Add characteristic polynomial
• Correct is_valid function to be False on a negative rank value
• Optimize circuits function (e.g. sage: %time C = matroids.CompleteGraphic(8).circuits() drops from 4m to 1m)
• Optimize broken_circuit_complex function
• Improve docstrings (1-line outputs and other changes)

⌛ Dependencies

• Depends on Fix noexcept clauses (#37560) #37667

[tscrim]

A lot of comments are the same, but I wanted to make a note of them (specifically the sorting). This could potentially generate a lot of merge conflicts, but hopefully that won't be a problem...

[mkoeppe]

Overall LGTM

[tscrim]

I have a few more changes to request; minor stuff basically.

[gmou3]

Thanks to both for the suggestions.

[tscrim]

Sorry, just a few more little things.

[tscrim]

Okay, thank you. Good to go.

[mkoeppe]

@gmou3 You may want to merge #37667 and update the changed pxd files

[github-actions]

Documentation preview for this PR (built with commit 236361b; changes) is ready! 🎉