backend: improve type operators support:

* add initial support for casts with generics;
* optimize out the casts to subtypes

Also add tests unchecked_cast{1,2}, the latter is disabled.

backend.native/tests/codegen/basics/unchecked_cast1.kt

@@ -0,0 +1,16 @@
+fun main(args: Array<String>) {
+    foo<String>("17")
+    bar<String>("17")
+    foo<String>(42)
+    bar<String>(42)
+}
+
+fun <T> foo(x: Any?) {
+    val y = x as T
+    println(y.toString())
+}
+
+fun <T> bar(x: Any?) {
+    val y = x as? T
+    println(y.toString())
+}

backend.native/tests/codegen/basics/unchecked_cast2.kt

@@ -0,0 +1,10 @@
+fun main(args: Array<String>) {
+    try {
+        val x = cast<String>(Any())
+        println(x.length)
+    } catch (e: Throwable) {
+        println("Ok")
+    }
+}
+
+fun <T> cast(x: Any?) = x as T