Implement OptionalCollector

[amaembo]

OptionalCollector is the collector specialization which return type is Optional<R> and it has methods which mimic the optional behavior (like orElse, orElseGet, get, map, filter, etc.). Should support both short-circuiting and non-short-circuiting collection.

[ThomasBlt]

@amaembo Isn't that solved by MoreCollectors.onlyOne / findFirst() / findAny() ?

[amaembo]

No, the idea was to create a separate public type OptionalCollector, so you can do something like MoreCollectors.onlyOne().orElse("foo") as a shortcut to Collectors.collectingAndThen(MoreCollectors.onlyOne(), opt -> opt.orElse("foo")). Not sure whether it worth the effort though.