Add coordinate abstractions

[JoeyBF]

I saw Jason had interesting code that handles bidegrees and bigraded vector spaces, so I ported it here with some modifications. Upstreamed from jassch/AdamsSSMasseyProductCalculator.

It remains to modify the rest of the code to use this machinery, but I wanted to get comments first.

[dalcde]

Haven't read through the code closely, but it is probably easier to make the s/t fields of Bidegree pub

[JoeyBF]

Haven't read through the code closely, but it is probably easier to make the s/t fields of Bidegree pub

I didn't want to do that because we might want to change the internal representation at some point

[JoeyBF]

I'll merge this PR next Friday if there aren't any more comments

[hoodmane]

Thanks @JoeyBF!

[JoeyBF]

No problem!