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

libraries/stdlib/test/collections/GroupingTest.kt

@@ -30,7 +30,7 @@ class GroupingTest {
         verifyGrouping(elements.asSequence().groupingBy(keySelector))
 
         val charSeq = "some sequence of chars"
-        verifyGrouping(charSeq.groupingBy { it.toInt() }, charSeq.toList(), charSeq.map { it.toInt() })
+        verifyGrouping(charSeq.groupingBy { it.code }, charSeq.toList(), charSeq.map { it.code })
     }
 
     // aggregate and aggregateTo operations are not tested, but they're used in every other operation