Calling Kotlin from C++ runtime (#107)

* backend: support exporting Kotlin functions for C++ runtime

* runtime: implement several functions in Kotlin

* Throw{NullPointer,ArrayIndexOutOfBounds,ClassCast}Exception

* runtime: add literal declared in Kotlin as empty string

Also
* add corresponding test
* use empty string literal in strdedup2
  (however keep it disabled)

* fixup! runtime: add literal declared in Kotlin as empty string

* Review feedback: add more TheEmptyString() usages

Also improve string index checks in runtime

backend.native/tests/build.gradle

@@ -323,6 +323,11 @@ task tostring2(type: RunKonanTest) {
     source = "runtime/basic/tostring2.kt"
 }
 
+task empty_substring(type: RunKonanTest) {
+    goldValue = "\n"
+    source = "runtime/basic/empty_substring.kt"
+}
+
 task array0(type: RunKonanTest) {
     goldValue = "5\n6\n7\n8\n9\n10\n11\n12\n13\n"
     source = "runtime/basic/array0.kt"
@@ -395,7 +400,9 @@ task strdedup1(type: RunKonanTest) {
 }
 
 /*
-TODO: enable after supporting Unit object
+TODO: string deduplication across several components seems to require
+linking them as bitcode modules before translating to machine code.
+
 task strdedup2(type: RunKonanTest) {
     goldValue = "true\ntrue\n"
     source = "datagen/literals/strdedup2.kt"

backend.native/tests/datagen/literals/strdedup2.kt

@@ -1,6 +1,6 @@
 fun main(args : Array<String>) {
-    val str1 = "kotlin.Unit"
-    val str2 = Unit.toString()
+    val str1 = ""
+    val str2 = "hello".subSequence(2, 2)
     println(str1 == str2)
     println(str1 === str2)
 }

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

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