RFR: JDK-8283416: Update java.lang.invoke.MethodHandle to use sealed classes

[Joe Darcy]

Small refactoring to use sealed classes in the MethodHandle implementation hierarchy.

DelegatingMethodHandle is non-sealed rather than sealed since it has two subclasses, one defined as a nested class and only a separate type in the same package. The permits clause does not allow listed those two kinds of subclasses.

Please also review the corresponding CSR https://bugs.openjdk.java.net/browse/JDK-8283434

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Commit messages:
 - JDK-8283416: Update java.lang.invoke.MethodHandle to use sealed classes

Changes: https://git.openjdk.java.net/jdk/pull/7881/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7881&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283416
  Stats: 16 lines in 5 files changed: 2 ins; 0 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7881.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7881/head:pull/7881

PR: https://git.openjdk.java.net/jdk/pull/7881