Fixed Equals method for transformers

[bahusoid]

Fixed possible equality issue:

public class MyTransformer: DistinctRootEntityResultTransformer {}
var myTransformer = new MyTransformer();
var distinctTransformer = new DistinctRootEntityResultTransformer();
myTransformer.Equals(distinctTransformer); // true. Should be false
distinctTransformer.Equals(myTransformer); // false

Also reuse instances when possible

[bahusoid]

This custom class also provides hashcode collision with current implementaion. Not sure why TweakHashcode was required

[fredericDelaporte]

See #564, it is required to check that two different result transformers will not be considered equal if they happen to have the same hashcode, which was NH-3957 bug. The collision for different instances of the same transformer is intentional and mandatory since they are considered equal.

Removing the tweak causes the Inequality suffixed tests to no more test the NH-3957 bug. You should change them to compare with the corresponding Custom descendant you have added. That is still a step away from what was NH-3957, but I do not see any other solution with the Instance optimization. NH-3957 was a random hashcode collision between different unrelated transformers causing them to wrongly consider themselves equals.

[bahusoid]

it is required to check that two different result transformers will not be considered equal if they happen to have the same hashcode,.. Removing the tweak causes the Inequality suffixed tests to no more test the NH-3957 bug

But I'm already checking Inequality:
Assert.False(custom.Equals(transf1));
Assert.False(transf1.Equals(custom));

I've just put those checks in the wrong tests.

That is still a step away from what was NH-3957

I still see no difference between inequality check for custom implemented classes with collisions to existing ones ( custom and transf1) and TweakHashcode hack for existing classes. IMHO they still fully cover the case:

that two different result transformers will not be considered equal if they happen to have the same hashcode