Refactor dispatch2

[madsmtm]

Refactor dispatch2 to use the DispatchRetained<T> pattern instead of types owning the retain count internally. This should allow us to use Dispatch types in the auto-generated headers. See #681 (comment) for why we don't use a more general approach to these *Retained types.

I've also decided to prefix types with "Dispatch", to better match Swift, and to allow easily glob-importing from dispatch2 without fear that the symbols will clash with something else. This isn't that "rusty", but it plays much nicer with the rest of the objc2 ecosystem, and matches what Swift does.

It's also clearer at the usage-site. Compare:

// Hmm, what queue are we referring to?
Queue::main().exec_sync(|| ...);

// Ah, it's a Dispatch queue.
DispatchQueue::main().exec_sync(|| ...);

See #681 for further discussion about this refactor.

Part of #77. Unblocks #710.