Automatic memory management of blocks

[madsmtm]

It would be nice if we could get automatic memory management for blocks as well, instead of having to specify returned blocks as *mut Block<dyn Fn()>, and then converting those with RcBlock::copy.

If I'm reading the ARC spec right, it seems like blocks are actually autoreleased according to (mostly) the same rules as other Objective-C objects, and that we need to use objc_retainAutoreleasedReturnValue to access returned block pointers?

A few examples of more complex memory-management:

• -[NSBackgroundActivityScheduler scheduleWithBlock:], an example of a block taking a block as a parameter, which is kinda interesting.
• NSProgressPublishingHandler, an example of a block returning a block.
• -[NSError userInfoValueProviderForDomain:], an example of a method returning a block.

Also, should blocks be allowed in Id?