Add String.subSequence implementation. (#61)

backend.native/tests/build.gradle

@@ -198,6 +198,11 @@ task tostring0(type: RunKonanTest) {
     source = "runtime/basic/tostring0.kt"
 }
 
+task tostring1(type: RunKonanTest) {
+    goldValue = "ello\n"
+    source = "runtime/basic/tostring1.kt"
+}
+
 task array0(type: RunKonanTest) {
     goldValue = "5\n6\n7\n8\n9\n10\n11\n12\n13\n"
     source = "runtime/basic/array0.kt"

backend.native/tests/runtime/basic/tostring1.kt

@@ -0,0 +1,4 @@
+fun main(args : Array<String>) {
+    val hello = "Hello world"
+    println(hello.subSequence(1, 5).toString())
+}