Implementation plan for powmod

[gwhitney]

• [x] Gather any existing math utilities into their own module, and add a powmod(x, y, m) function that computes the mod-m residue of x^y reasonably efficiently. Presumably this would be implemented for bigints x,y,m -- is there a need for any of them to be any other type?
• [ ] Modify SequenceFormula to evaluate the sequence formula in a context in which it has access to powmod (and document this in the on-line documentation)

Does that sound like the right approach for now? Let me know and let me know about the types, and if so, I will put this on my to-do list after Zulip.

[katestange]

Powmod using square-and-multiply should have no problem handling bigints for all the inputs; it's a pretty straightforward polynomial-time thing in the number of digits. I think bigints is appropriate.

Yay for getting SequenceFormula to do powmod! Go for it!

[gwhitney]

This is bogged down in the possibly eternal wait for mathjs to properly support bigints...