Rewrite ProgressionUtil to Kotlin, move to "kotlin.internal"

Use reflection in ProgressionUtilTest instead of calling methods on the class
from the class path, because in the latter case we were just testing
ProgressionUtil from bootstrap runtime (which, presumably, always works
correctly since it's a bootstrap distribution)

With this change, compilable built-ins (under core/builtins/src) are fully
written in Kotlin

generators/src/org/jetbrains/jet/generators/builtins/progressionIterators.kt

@@ -33,7 +33,7 @@ fun integerProgressionIterator(kind: ProgressionKind): String {
 
     return """class ${t}ProgressionIterator(start: $t, end: $t, val increment: $incrementType) : ${t}Iterator() {
     private var next = start$toInt
-    private val finalElement: $t = ProgressionUtil.getProgressionFinalElement(start$toInt, end$toInt, increment)$toType
+    private val finalElement: $t = getProgressionFinalElement(start$toInt, end$toInt, increment)$toType
     private var hasNext: Boolean = if (increment > 0) start <= end else start >= end
 
     override fun hasNext(): Boolean = hasNext
@@ -69,7 +69,7 @@ fun floatingPointProgressionIterator(kind: ProgressionKind): String {
 
 class GenerateProgressionIterators(out: PrintWriter) : BuiltInsSourceGenerator(out) {
     override fun generateBody() {
-        out.println("import jet.runtime.ProgressionUtil")
+        out.println("import kotlin.internal.getProgressionFinalElement")
         out.println()
         for (kind in ProgressionKind.values()) {
             if (kind != FLOAT && kind != DOUBLE) {