RFR: 8341260: Add Float16 to jdk.incubator.vector [v12]

[Joe Darcy]

> Port of Float16 from java.lang in the lworld+fp16 branch to jdk.incubabor.vector.

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 21 additional commits since the last revision:

 - Test update and minor bug fix.
 - Merge branch 'master' into JDK-8341260
 - Add Float16 -> string conversion from Raffaello.
 - Merge branch 'master' into JDK-8341260
 - Merge branch 'master' into JDK-8341260
 - Respond to review feedback.
 - Add support for proper String -> Float16 conversion.
 - Merge branch 'master' into JDK-8341260
 - Improve specification per code review feedback.
 - Add tests, improve hashing spec.
 - ... and 11 more: https://git.openjdk.org/jdk/compare/f69b3a3a...8af04c4d

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21574/files
  - new: https://git.openjdk.org/jdk/pull/21574/files/7c50bbe0..8af04c4d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21574&range=11
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21574&range=10-11

  Stats: 205741 lines in 2051 files changed: 129905 ins; 52738 del; 23098 mod
  Patch: https://git.openjdk.org/jdk/pull/21574.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21574/head:pull/21574

PR: https://git.openjdk.org/jdk/pull/21574