Optimization: Don't store descendants of BasicObject and friends

[amomchilov]

Every single class in the system is a descendant of BasicObject. We don't need to spend memory to remember them all, we can just look at the list of all constants.

Similarly, almost every BasicObject is also an Object (which includes Kernel). Module/Class would also have particularly large descendants lists. We should investigate optimizing these.

For what it's worth, "What is every descendant of Class in my system?" is not a particularly common/useful question, so it's okay if it's slower than queries.