RFR: JDK-8280168 Add Objects.toDefaultString

[Joe Darcy]

While it is strongly recommend to not use the default toString for a class, at times it is the least-bad alternative. When that alternative needs to be used, it would be helpful to have the implementation already available, such as in Objects.toDefaultString(). This method is analagous to System.identityHashCode.

Please also review the CSR: https://bugs.openjdk.java.net/browse/JDK-8280184

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Commit messages:
 - JDK-8280168 Add Objects.toDefaultString

Changes: https://git.openjdk.java.net/jdk/pull/7139/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7139&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280168
  Stats: 35 lines in 2 files changed: 32 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7139.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7139/head:pull/7139

PR: https://git.openjdk.java.net/jdk/pull/7139