RFR: JDK-8280168 Add Objects.toDefaultString [v6]

[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

Joe Darcy has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'master' into JDK-8280168
 - Appease jcheck.
 - Add toIdentityString
 - Respond to review feedback to augment test.
 - Respond to review feedback.
 - JDK-8280168 Add Objects.toDefaultString

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7139/files
  - new: https://git.openjdk.java.net/jdk/pull/7139/files/0f6f0786..0a4d8a4f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7139&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7139&range=04-05

  Stats: 6376 lines in 249 files changed: 4107 ins; 1549 del; 720 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