[lworld+fp16] RFR: 8329817: Augment prototype Float16 class

[Joe Darcy]

Sketch of a more full-featured API for Float16.

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Commit messages:
 - JDK-8329817: Augment prototype Float16 class

Changes: https://git.openjdk.org/valhalla/pull/1117/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1117&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329817
  Stats: 512 lines in 1 file changed: 491 ins; 6 del; 15 mod
  Patch: https://git.openjdk.org/valhalla/pull/1117.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1117/head:pull/1117

PR: https://git.openjdk.org/valhalla/pull/1117