Bridges (#240)

* - bridges building
- bridges support in vtable and methods table

* FunctionDescriptor.signature fix + some assertions

* tests

* refactoring

* refactoring

* removed debug output

* refactoring

* review fixes

* implemented bridge construction for parameters

* tests

* refactoring

* removed custom box/unbox

* added bridge for extension receiver

backend.native/tests/codegen/bridges/test2.kt

@@ -0,0 +1,17 @@
+// vtable call, bridge inherited
+open class A {
+    open fun foo(): Any = "A"
+}
+
+open class C : A() {
+    override fun foo(): Int = 42
+}
+
+open class D: C()
+
+fun main(args: Array<String>) {
+    val c = D()
+    val a: A = c
+    println(c.foo().toString())
+    println(a.foo().toString())
+}