Add some string operations, refactor runtime. (#249)

backend.native/tests/runtime/text/string0.kt

@@ -0,0 +1,9 @@
+fun main(args: Array<String>) {
+    val str = "hello"
+    println(str.equals("HElLo", true))
+    val strI18n = "Привет"
+    println(strI18n.equals("прИВет", true))
+    println(strI18n.toUpperCase())
+    println(strI18n.toLowerCase())
+    println("пока".capitalize())
+}