8252827: Caching Integer.toString just like Integer.valueOf

Hello, does the enhancement proposed in [1] make sense, both today and when wrappers will be migrated to primitive classes? If so, it should be rather simple to add it and I could prepare a PR. If not, the issue might perhaps be closed. Greetings Raffaello ---- [1] https://bugs.openjdk.java.net/browse/JDK-8252827