Bidirectional type inference in is expressions

[kodek16]

Currently in the expression a is b only b may have a context-dependent type (which is inferred from the type of a). This covers the more common form p is null for example, but null is p causes an ambiguous type error.

While having null on the left side looks a bit weird, it is still a valid use case (and there can be other ambiguous-type expressions as well). Need to find a way to enable type inference in both directions.