GROOVY-9337: add extension methods for Optional, OptionalInt, et al.

[eric-milles]

https://issues.apache.org/jira/browse/GROOVY-9337

Adds:

• get() to OptionalInt, OptionalLong, OptionalDouble
• filter(Class) to Optional
• filter([Int|Long|Double]Predicate) to OptionalInt, OptionalLong, OptionalDouble
• mapToObj([Int|Long|Double]Function) to OptionalInt, OptionalLong, OptionalDouble
• mapToInt(ToIntFunction) to Optional
• mapToLong(ToLongFunction) to Optional
• mapToDouble(ToDoubleFunction) to Optional
• stream() to Optional -- exists as normal method since Java 9

No stream() yet for OptionalInt, OptionalLong, OptionalDouble because it is unclear if return should be Stream or IntStream. Looking into DGSMs IntStream.of(OptionalInt) and Stream.of(OptionalInt) to support both options. Also, as with Optional#stream, OptionalInt, etc. have a stream() method since Java 9.

There are some additional items mentioned in GROOVY-9337; I haven't completed research for them yet.

[daniellansun]

+0
I'm not sure whether we should add new features to 3.0.0 at this moment as 3.0.0 GA will be out very soon.

[daniellansun]

As nobody rejects the PR in the past 72 hours, merged. Thanks.