Initial transition to objc2

[madsmtm]

Move to objc2, my fork of objc which adds a lot of things, in particular macros for proper memory management, and a lot of fixed soundness issues.

Split out from of #2427.

Concrete improvements in objc2 which can be seen in this PR:

• msg_send! can now return bool without special handling.
• Types in msg_send! are now required to be Encode. This helped catching a lot of undefined behaviour, which are fixed in Fix some invalid msg_send! usage #2138 and iOS: Fix a few instances of UB #2428.

Checklist:

• Tested on all platforms changed
  • macOS 10.14
  • iOS simulator (can't get log output here though, so a bit harder)
  • iOS 9.3.6 (iPad mini 1st gen)
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

[madsmtm]

This is only in the interim; once #2427 is fully merged, the need for this will be gone

[madsmtm]

Note that objc2 doesn't have a MSRV policy yet, see madsmtm/objc2#203.

But since we use a pinned dependency, this shouldn't be a problem (yet).

[kchibisov]

I don't have that much to say, given that I have no clue about the objc internals. The changes look good, some minor notes.

[kchibisov]

This has nothing to do with that PR, but could libdispatch used to drop from the main thread? So we won't have runtime panics like that? Though I think the issue that you can't use that from the other thread as well?

It just all looks wrong to me. We say that winit window is Send and you can get MonitorHandle from the Window(owning one iirc), but when you drop it you'll panic?

[madsmtm]

There is so much thread-unsafety in our macOS (and probably also iOS, but less well-versed in that) implementation, really, you have no idea!

Anyway, to answer your question: Yes, we could and should use libdispatch to do things on the main thread, both here and in a lot of other places. I'm in the process of exploring solutions, have focused on objc2 stuff first though.