Add set operator to make the work with StringBuilder more list-like.

Make StringBuilder.set jvm-only.
Move jvm-specific test.

libraries/stdlib/test/text/StringBuilderJVMTest.kt

@@ -0,0 +1,15 @@
+package test.text
+
+import kotlin.*
+import kotlin.test.*
+import org.junit.Test as test
+
+class StringBuilderJVMTest() {
+    @test fun getAndSetChar() {
+        val sb = StringBuilder("abc")
+        sb[1] = 'z'
+
+        assertEquals("azc", sb.toString())
+        assertEquals('c', sb[2])
+    }
+}