New J2K: separate nullability inference from common one & nullability bug fixes

It will be needed for structure mutability inference

#KT-21467 fixed
#KT-32609 fixed
#KT-32572 fixed
#KT-24677 fixed

nj2k/testData/inference/common/lambdaAssign.kt

@@ -0,0 +1,8 @@
+fun test() {
+    val x: /*T5@*/Function2</*T2@*/Int, /*T3@*/String, /*T4@*/Boolean> = { a: /*T0@*/Int, b: /*T1@*/String -> true/*LIT*/ }/*Function2<T0@Int, T1@String, T6@Boolean>*/
+}
+
+//LOWER <: T6 due to 'RETURN'
+//T2 <: T0 due to 'INITIALIZER'
+//T3 <: T1 due to 'INITIALIZER'
+//T6 <: T4 due to 'INITIALIZER'