Unify equality checks on (parse) trees.

[PieterOlivier]

The interpreter has code to compare two (parse) trees in ConcreteSyntaxResult. This code works on IConstructor instances and assumes they are ITree instances
But ITree subclasses already have equals methods implemented that are much more efficient.

We should unify these implementations but keep in mind that we need to be able to compare with and without ignoring layout and keyword fields.