Try to distinguish between boxed and primitive types by nullability

[Jeffset]

In order to correctly override methods in Java code in KSP mode, Yatagan uses KSP's experimental APIs to obtain JVM signatures to distinguish between boxed/unboxed java types (e.g. java.lang.Integer vs int). As per https://github.com/google/ksp/issues/870#issuecomment-1268030388, there can be no need for that, nullability info alone may be sufficient to make the distinction.

Let's give it a shot. If it's indeed possible, removing cumbersome JVM-signature obtaining/parsing will greatly simplify the implementation while likely improving performance.