accessor for private constructors #KT-2716 fixed

also on the way done small refactoring of JetTypeMapper
- few code duplicates replace by methods
- mapToCallableMethod(ConstructorDescriptor) introduced

compiler/testData/codegen/regressions/kt2716.kt

@@ -0,0 +1,11 @@
+package someTest
+
+public class Some private(val v: String) {
+    class object {
+        public fun init(v: String): Some {
+            return Some(v)
+        }
+    }
+}
+
+fun box() = Some.init("OK").v