Move to objc2

[madsmtm]

A sneak-peak at the improvements I've been working on for iOS and macOS.

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.

When done, this should vastly improve the safety and ease-of-maintenance of the iOS and macOS platforms. Opening this PR to have something to refer to, but will split it up into smaller parts when I'm ready to actually get it merged.

• 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

[notgull]

Should we enable the "exception" feature? It seems unsound got exceptions to unwind into Rust.

[madsmtm]

Should we enable the "exception" feature? It seems unsound got exceptions to unwind into Rust.

The feature flag is called catch-all if you want every msg_send! to be wrapped in a @try/@catch.

Re soundness: That feature is just as unsound as letting exceptions unwind into Rust, since our message sending is implemented in Rust. extern "C-unwind" would need to be stable before it is sound (can be tried with objc2's unstable-c-unwind feature flag), but that makes the point moot as letting exceptions just unwind into Rust also becomes sound.

Practically, I haven't enabled it because exceptions are rare, the user-experience approximately the same, and enabling it requires running clang which is inconvenient for some users.

Note that people can still enable it if they want by depending on objc2 and enabling the feature themselves

[madsmtm]

This is very close to where I want it to be, so I'll start opening smaller PRs.

[madsmtm]

This is finished, the remaining parts are in #2465 and #2462