RFR: 8279986: methods Math::asXExact for safely checked primitive casts

[Raffaello Giulietti]

Add a family of "safe" cast methods.

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Commit messages:
 - 8279986: methods Math::asXExact for safely checked primitive casts
 - 8279986: methods Math::asXExact for safely checked primitive casts
 - 8279986: methods Math::asXExact for safely checked primitive casts

Changes: https://git.openjdk.java.net/jdk/pull/8548/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8548&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279986
  Stats: 615 lines in 2 files changed: 609 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8548.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8548/head:pull/8548

PR: https://git.openjdk.java.net/jdk/pull/8548