Expressions use multiple inheritance for Constant, Variable, Operation

[ekilmer]

More minimal PR after breaking it out of #1729

Removes some duplicate code

NOTE: This branch will be rebased onto master after expression-keyword-only-init branch/PR #2395 is merged

[ehennenfent]

I don't see any code problems with this PR, and I think the technique of dynamically creating __slots__ for multiply-inherited subclasses is really exciting. That said, since this more or less completely reverts #1635, I'd like to get your thoughts on a couple of matters:

First, changing the inheritance structure reduces the amount of duplicated code, but means we have to do things like the __xslots__ trick to get good performance. Are we certain the ergonomics of one approach are better than the other?

Second, how much does this change affect performance? #1635 gave us a modest improvement, but not large enough to be noticeable without measuring, so we should measure this branch as well to see how it stacks up.

[ekilmer]

After testing with multiple-styles, as was done in #1635 I saw a slight slowdown with the use of multiple inheritance. Testing with a microbenchmark that instantiates the Constant and Variable subclass objects also reveals some performance penalties for the multiple inheritance. I didn't test with Operation subclasses, but I suspect it to be a similar story and also more greatly contribute to a performance drop, since Operations are used more in expressions.

I think the test_Api test we have to do sanity checking is good enough, and I don't really expect too much churn on these objects to make maintenance unmanageable, so I will close this PR.