[JS IR BE] hashCode, toString, number conversion support

js/js.translator/testData/box/builtins/toString.kt

@@ -0,0 +1,20 @@
+// EXPECTED_REACHABLE_NODES: 1117
+package foo
+
+class A {
+    override fun toString() = "42"
+}
+
+class B
+
+fun box(): String {
+    assertEquals(A().toString(), "42")
+    assertEquals(B().toString(), "[object Object]")
+    assertEquals(js("\"\"").toString(), "")
+    assertEquals(js("123").toString(), "123")
+    assertEquals(123.toString(), "123")
+    assertEquals("123".toString(), "123")
+    assertEquals((123 as Any).toString(), "123")
+    assertEquals(null.toString(),  "null")
+    return "OK"
+}