[lworld+fp16] RFR: 8339252: Add method float16Value() to BigInteger and BigDecimal [v4]

[Raffaello Giulietti]

> This patch adds `float16Value()` to `BigInteger` and `BigDecimal`, as well as `Float16.valueOf(BigDecimal)`.

Raffaello Giulietti has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - Merge branch 'lworld+fp16' into 8339252
 - Clearer code for fullFloat16Value().
 - More clarification in a comment.
 - 8339252: Add method float16Value() to BigInteger and BigDecimal
 - Added valueOf(BigDecimal).

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Changes: https://git.openjdk.org/valhalla/pull/1228/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1228&range=03
  Stats: 310 lines in 5 files changed: 301 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/valhalla/pull/1228.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1228/head:pull/1228

PR: https://git.openjdk.org/valhalla/pull/1228