Equality: less automatic simplification and fix 'Eq(p, 0)' for p positive

update: no longer on top of #7959 and #7838.
Isn't there something like a `compatible_assumptions` test? e.g. lhs might be positive and rhs negative so this can't be equal. Lacking something else, this is what comes to mind:

```
def compatible(a, b):
    if a.assumptions0 == b.assumptions0:
        return True
    da = a.assumptions0
    db = b.assumptions0
    ka = set(da.keys())
    kb = set(db.keys())
    both = ka & kb
    for k in both:
        va = da[k]
        vb = db[k]
        if None not in (va, vb) and va != vb:
            return False
    for do in range(2):
        ina = dict([(k, da[k]) for k in ka - kb if da[k] is not None])
        db.update(ina)
        try:
            Dummy(**db)
        except InconsistenAssumptions:
            return False
        if do == 0:
            for k in ina:
                db.pop(k)
            ka, kb = kb, ka
            da, db = db, da
    return True

>>> a = Dummy(real=True)
>>> b = Dummy(complex=True)
>>> compatible(a, b)
True
>>> b=Dummy(imaginary=1)
>>> compatible(a, b)
False
>>> b=Dummy(positive=True)
>>> compatible(a, b)
True
```