Suggested changes to the old assumptions

These are the changes that I think should be made to the old assumptions. 

Because these changes are likely to be contentious, I have split each change into a separate commit. I have also only made the change to assumptions.py. I don't want to waste time propagating the change throughout the code-base until we agree that the change should be made. 

If you think it would be even better to make each change a separate pull request, let me know. I don't know if that will make things easier or harder when propagating the changes throughout the code-base, however. 
> This matches all the other "non" assumptions

I don't like this silent "binding" of "non"-prefix and reals.  Someday we may want introduce a new "nonfoo"-style assumption for complex numbers or something else.

And nonzero - could be such assumption.  We might want to check is_nonzero property for complex divisors as well.  I think, this is most important use case for is_nonzero.

> It also matches the new assumptions.

Well, new assumptions could be wrong)