RFR: 8186085: (opt) add filter(), flatMap(), and map() methods to OptionalDouble/Int/Long

[Kartik Ohri]

Hi all,
This PR intends to add filter, map and flatMap methods to the Optional classes for primitives. The rationale is consistency with the Optional class for objects and user convenience.
Thanks.
Regards,
Kartik

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Commit messages:
 - 8186085: (opt) add filter(), flatMap(), and map() methods to OptionalDouble/Int/Long

Changes: https://git.openjdk.java.net/jdk/pull/857/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=857&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8186085
  Stats: 336 lines in 6 files changed: 336 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/857.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/857/head:pull/857

PR: https://git.openjdk.java.net/jdk/pull/857