manually revert 4346424be94ff3e0a4a4fbfc5a6f35a3a0009d5a

allow var('x', zero=True) -- there are already many ways to work around the current return of 0. One advantage of having this is that then one can work with a zero expression without using an unevaluated expression:

```
>>> var('z',zero=1)
z
>>> var('x',bounded=1)
x
>>> z*x  # doesn't evaluates
x*z
>>> _.is_zero
True
>>> 0*x  # evaluates
0
```