RFR: 8316493: Make immutable maps @ValueBased [v2]

[Per Minborg]

> This PR outlines a solution for making immutable maps `@ValueBased` by removing cacheing of certain values in `AbstractMap`.
> 
> By removing these caching fields in `AbstractMap`, we can make the immutable maps `@ValueBased` and at the same time, performance is likely improved because the JVM is probably able to optimize away object creation anyway via escape analysis. Also, all maps will occupy less space as we get rid of a number of objects and references stored for each map.
> 
> We need to benchmark this solution to better understand its implications.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Remove redundant impl spec parts

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15614/files
  - new: https://git.openjdk.org/jdk/pull/15614/files/f0410ee3..2cb090b6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15614&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15614&range=00-01

  Stats: 8 lines in 1 file changed: 0 ins; 8 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15614.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15614/head:pull/15614

PR: https://git.openjdk.org/jdk/pull/15614