Fixes to handling of boxed types

Boxed types and primitive types should cooperate properly now with
primitive types in all basic cases. Added tests that verify this.

tests/testData/ast/boxedType/file/Boxing.jav

@@ -0,0 +1,14 @@
+import java.util.ArrayList;
+import java.util.List;
+
+class Boxing {
+    void test() {
+        Integer i = 0;
+        Number n = 0.0f;
+        i = 1;
+        int j = i;
+        Integer k = i+2;
+        i = null;
+        j = i;
+    }
+}

tests/testData/ast/boxedType/file/Boxing.kt

@@ -0,0 +1,12 @@
+import java.util.ArrayList
+open class Boxing() {
+open fun test() : Unit {
+var i : Int? = 0
+var n : Number? = 0.0.toFloat()
+i = 1
+var j : Int = i!!
+var k : Int? = i!! + 2
+i = null
+j = i!!
+}
+}

tests/testData/ast/file/comp/intNullability.kt

@@ -4,6 +4,6 @@ open class Test() {
 open fun test() : Unit {
 var i : Int? = 10
 var j : Int? = 10
-System.out?.println(i + j)
+System.out?.println(i!! + j!!)
 }
 }

tests/testData/ast/issues/file/kt-794-mixed.kt

@@ -4,6 +4,6 @@ open fun getInteger(i : Int?) : Int? {
 return i
 }
 open fun test() : Unit {
-var i : Int = getInteger(10)
+var i : Int = getInteger(10)!!
 }
 }