Add Dispatch / `libdispatch`

[madsmtm]

Reasoning:

• Better main thread support.
• objc2 encoding support (allowing use in framework crates).
• async support?
• Optimizations (such as the null-ptr optimization, and others).

Desired name: dispatch2, since it fits fairly well with the naming scheme (objc2, block2 and dispatch2 would all be successors to objc, block and dispatch, and would all be libraries internally, as compared to the rest of the crates we expose, which are all frameworks). I've asked for the name here, should remember to not close this issue until that repo is archived too.

Unsure how auto-generated we can feasibly be here? But the library is comparatively small compared to most frameworks, so it's probably possible to generate an initial starting point, and then refine later on.

Upstream dispatch issues:

• Add support for dispatch sources SSheldon/rust-dispatch#4
• Add support for QOS SSheldon/rust-dispatch#7
• would you pls add a demo sample run on iOS ? SSheldon/rust-dispatch#14
• Expose Queue's underlying dispatch_queue_t SSheldon/rust-dispatch#15
• Dispatching on main queue doesn't seem to actually run on main thread. SSheldon/rust-dispatch#20

Upstream dispatch pull requests to integrate:

• Add Dispatch sources SSheldon/rust-dispatch#6
• [WIP] support dispatch block/data/io objects SSheldon/rust-dispatch#11
• Add as_raw() methods for queues, groups and semaphores. SSheldon/rust-dispatch#17
• Implement unwind safety SSheldon/rust-dispatch#21
• Implemented GCD timer SSheldon/rust-dispatch#22

Integrate ideas from:

• dispatch
• fruity's dispatch
• dispatchr
• dispatch2
• cidre's dispatch

Other TODOs:

• Use in framework crates.
• Refactor such that we have less ugly types lying around.

[silvanshade]

I think this would be a good idea. Note that the types are needed for a full translation of the BackgroundTasks framework just added. I also find that I am using it quote a lot for working with the WebKit framework through icrate, though it's conceivable that many of those instances would go away if #279 happens.

[pronebird]

This would be incredibly useful and with the following pr being merged, it would enable a quite nice interop between ffi/objc/rust.