WIP: Remove the default "recursive __call__".

Now if one needs to use an expression as an operator, the `rcall` method should
be used. The use of the default `__call__` method was deemed too confusing for
novice users.

There are some failures in modules that I have not worked on. I can apply the change to them as well, and I will do it, I just want a quick confirmation by someone more knowledgeable.