RFR: JDK-8308040 Evaluate new public types in non-public classes

[Jim Laskey]

As reported by Marc Hoffman of javaalmanac.io, in a recent JDK 21 build there are new public types nested in non-public interfaces: 

java.lang.runtime.ReferenceKey.SoftKey 
java.lang.runtime.ReferenceKey.StrongKey 
java.lang.runtime.ReferenceKey.WeakKey 
java.util.Digits.DecimalDigits 
java.util.Digits.HexDigits 
java.util.Digits.OctalDigits

This PR moves the nested classes to neighbouring package private classes

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Commit messages:
 - Changes for ReferencedKeyMap
 - Changes for Digits

Changes: https://git.openjdk.org/jdk/pull/14039/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14039&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308040
  Stats: 989 lines in 10 files changed: 586 ins; 396 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/14039.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14039/head:pull/14039

PR: https://git.openjdk.org/jdk/pull/14039