Introducing racks, quandles and fixing minor bugs

[erzvtvhfm-fhjnyfxv]

This commit introduces racks and quandles. Besides that, it makes other theories consistent.

[andrejbauer]

Many thanks. I think there's some value in having, e.g., both a non-commutative and a commutative field, for comparison. But I like the idea that the non-efficient theories should be marked as such, althought perhaps in the comment rather than the filename.