RFR: JDK-8310913 Move ReferencedKeyMap to jdk.internal so it may be shared

[Jim Laskey]

java.lang.runtime.ReferencedKeyMap was introduced to provide a concurrent caching scheme for Carrier objects. The technique used is generally useful for a variety of caching schemes and is being moved to be shared in other parts of the jdk. The MethodType interning case is one example.

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Commit messages:
 - unneeded SuppressWarning
 - Wrong cast
 - Move jdk.internal.referencedkey to jdk.internal.util
 - Merge branch 'master' into 8310913
 - Update MethodType to use ReferencedKeySet
 - Initial commit for ReferencedKeyMap and ReferencedKeySet

Changes: https://git.openjdk.org/jdk/pull/14684/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14684&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310913
  Stats: 1391 lines in 12 files changed: 773 ins; 612 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/14684.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14684/head:pull/14684

PR: https://git.openjdk.org/jdk/pull/14684