Streamline EuclideanRing/Field.

This commit removes lcm() and gcd() from EuclideanRing, and adds
default implementations of quot() and mod() to Field. Since most
Field users won't need lcm/gcd implementations, it seemed better
to provide those implementations somewhere else.
Add some demo instances to src/test.

This helps show how instances might be defined, and ensures that
the library is relatively flexible and lightweight.

Currently there is no plan to ship default instances with the algebra
jar, since different libraries have different policies about how to
put implicit instances in scope.
Add 2.11.1 support, bump to 2.10.4.