RFR: JDK-8277520: Implement JDK-8 default methods for IdentityHashMap [v4]

[liach]

> Might need a CSR as now `computeIfAbsent` `computeIfPresent` `compute` `merge` would throw CME if the functions modified the map itself, and there are corresponding specification changes.

liach has updated the pull request incrementally with two additional commits since the last revision:

 - merge branch 'identityhashmap-bench' of https://github.com/liachmodded/jdk into identityhashmap-default
 - fix whitespace

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6532/files
  - new: https://git.openjdk.java.net/jdk/pull/6532/files/f6ee4fab..9fe578be

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6532&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6532&range=02-03

  Stats: 105 lines in 1 file changed: 0 ins; 0 del; 105 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6532.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6532/head:pull/6532

PR: https://git.openjdk.java.net/jdk/pull/6532