fix EqualsChecker for assignments

[Egor18]

Hi! I have found a bug, in EqualsChecker class, and this PR is a possible fix.
The problem is that .equals method called on CtAssignment and CtOperatorAssignment (which extends CtAssignment) returns true, once rhs and lhs statements are the same. But this is not correct.
For example, statements x = 42 and x += 42 are not the same thing at all.
To fix that, I implemented visitCtAssignment method.
Also, I provided some tests (second assert fails, without this fix).

[monperrus]

Thanks a lot.

I wonder whether we don't have similar bugs due to covariance / contravariance in the equality visitor.