[lworld+fp16] RFR: 8341245: Add 'shortest length' toString() to Float16

[Raffaello Giulietti]

Add `Float16.toString()` with the same 'shortest length' properties as of `[Double|Float].toString()`.

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Commit messages:
 - 8341245: Add 'shortest length' toString() to Float16

Changes: https://git.openjdk.org/valhalla/pull/1260/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1260&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341245
  Stats: 679 lines in 5 files changed: 674 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/valhalla/pull/1260.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1260/head:pull/1260

PR: https://git.openjdk.org/valhalla/pull/1260