blocks:

• SR-72 NSNumber is missing much parity

Issue Description:

When creating clusters like NSString, NSNumber NSDictionary etc we have to pollute the subclasses with ivars and we don't have uniqueness like [NSNumber numberWithBool:]'s return value being identical or NSNull being identical to all creations.

This leads to either poor performance and memory thrashing, or it will lead to divergent interfaces that make the swift-corelibs-foundation behave differently than the darwin objc version.

Proposals have been floated around for this and it seems amenable; however it would be nice to pick those up in as official proposals to un-block the development of Foundation with swift.