"field": synthetic variable creation in accessors, codegen changed accordingly, a set of tests, relevant code fix (j2k)

compiler/testData/codegen/box/properties/field.kt

@@ -0,0 +1,10 @@
+var my: String = ""
+    get() = field + "K"
+    set(arg) {
+        field = arg
+    }
+
+fun box(): String {
+    my = "O"
+    return my
+}