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

update: no longer on top of #7959 and #7838.
There are *lots* of ways things can be incompatible, but those should already be tested in is_zero so it's hard to know what tests should actually be here once is_zero is used to test lhs - rhs. I wouldn't include any tests for Eq other than a trivial one to test coverage but I would include a clear not in the code that tells why is_zero is being used and why it should detect all incompatibilities between lhs and rhs. just include one esoterically/less obvious incompatible test like `Eq(Dummy('integer'), Dummy('integer'=False, real=True, zero=False))` and include a note that says that any incompatibility should be detected by is_zero.