RFR: JDK-8170945: Collectors$Partition should override more Map methods [v2]

[Viktor Klang]

> Adds overrides for common Map operations to avoid having to allocate the entrySet for Collectors$Partition maps.

Viktor Klang has updated the pull request incrementally with one additional commit since the last revision:

  Let auto-boxing do its thing for Collectors.get(key)

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13347/files
  - new: https://git.openjdk.org/jdk/pull/13347/files/97a48145..15a57ca1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13347&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13347&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13347.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13347/head:pull/13347

PR: https://git.openjdk.org/jdk/pull/13347