Deprecate Char-to-Number conversions in stdlib (JVM and JS)

- Int.toChar was left non-deprecated because the replacement is not intrinsic yet.
- Number.toChar was left non-deprecated because otherwise the deprecation propagates to the override, Int.toChar.

KT-23451

generators/builtins/progressionIterators.kt

@@ -27,7 +27,7 @@ fun integerProgressionIterator(kind: ProgressionKind): String {
     val incrementType = progressionIncrementType(kind)
 
     val (toInt, toType) = when (kind) {
-        CHAR -> ".toInt()" to ".to$t()"
+        CHAR -> ".code" to ".toChar()"
         else -> "" to ""
     }
 
@@ -36,9 +36,9 @@ fun integerProgressionIterator(kind: ProgressionKind): String {
  * @property step the number by which the value is incremented on each step.
  */
 internal class ${t}ProgressionIterator(first: $t, last: $t, val step: $incrementType) : ${t}Iterator() {
-    private val finalElement = last$toInt
+    private val finalElement: $incrementType = last$toInt
     private var hasNext: Boolean = if (step > 0) first <= last else first >= last
-    private var next = if (hasNext) first$toInt else finalElement
+    private var next: $incrementType = if (hasNext) first$toInt else finalElement
 
     override fun hasNext(): Boolean = hasNext