Consider changing Elemwise and CAReduce to abstract classes

[brandonwillard]

If we change Elemwise and CAReduce to abstract classes, we can start treating Elemwise + scalar Op combinations as types and more easily distinguish them and generalize their handling in code.

Our Elemwise and CAReduce instances are already acting as singletons, and there isn't a critical use for "dynamically" generated Elemwise instances, so the shift to explicit subclasses won't change much. If anything, it will simplify a lot of our code, because we would be able to use isinstance(op, Add) and cover every Elemwise(add) that we currently need to check with isinstance(op, Elemwise) and op.scalar_op == add and the like. It would also allow us to fix some really poor logic like the type(node.op) == CAReduce used here, and the brittle, convoluted type checking used here.

The changes involved in this mostly involve replacing/changing the use of scalar_elemwise. Instead, where scalar_elemwise is currently used to create an Elemwise instance, we would create an Elemwise subclass and its single instance.