RFR: 8205592: BigDecimal.doubleValue() is depressingly slow

[Raffaello Giulietti]

A reimplementation of `BigDecimal.[double|float]Value()` to enhance performance, avoiding an intermediate string and its subsequent parsing on the slow path.

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Commit messages:
 - 8205592: BigDecimal.doubleValue() is depressingly slow

Changes: https://git.openjdk.org/jdk/pull/9410/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9410&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8205592
  Stats: 585 lines in 2 files changed: 545 ins; 8 del; 32 mod
  Patch: https://git.openjdk.org/jdk/pull/9410.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9410/head:pull/9410

PR: https://git.openjdk.org/jdk/pull/9410