Optimize keySet / values for Map1

[kilink]

Override keySet and values methods for ImmutableCollections.Map1 to return singleton collections.

Map1 inherits from the keySet / values methods from AbstractMap, which both cache and return implementations based on the entrySet iterator. Provide overrides to return specialized implementations for singletons (Set.of / List.of).

Note: Collections.singletonMap already provides similar overrides.

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[liach]

#18522 has the same optimizations. I personally believe that patch is more suitable because:

1. Per 8316493: Remove the caching fields in AbstractMap #15614, we are seeking to remove these cache fields for project valhalla to make these immutable collection value objects, so we won't use the cache fields.
2. 8328821: Map.of() derived view collection mutators should throw UnsupportedOperationException #18522 considers the behavioral changes between the key set and the values collections for Collection::remove(Object) when the argument object is absent (false return versus throwing UOE) or null (false return versus throwing UOE/NPE) more comprehensively.

[kilink]

#18522 has the same optimizations. I personally believe that patch is more suitable because:

1. Per 8316493: Remove the caching fields in AbstractMap #15614, we are seeking to remove these cache fields for project valhalla to make these immutable collection value objects, so we won't use the cache fields.
2. 8328821: Map.of() derived view collection mutators should throw UnsupportedOperationException #18522 considers the behavioral changes between the key set and the values collections for Collection::remove(Object) when the argument object is absent (false return versus throwing UOE) or null (false return versus throwing UOE/NPE) more comprehensively.

Thanks, I hadn't seen those. I'll close this in favor of those.