Fix arc borrow provenance of, and add a test that actually uses ArcBorrow

[steffahn]

This PR also had to remove the ArcBorrow::from_ref API since it was unconditionally unsound to ever use.

Further breakage is in the former Send/Sync implementation, as this fixes #66

Fixes #65.

This PR supersedes #64. Sorry for the long delay 😅

[Manishearth]

Thanks! CI fails unfortunately.

[steffahn]

I missed testing with --all-features :-)

Should work now…

[steffahn]

remove the ArcBorrow::from_ref API since it was unconditionally unsound to ever use

Actually, maybe not all usages would be unsound, but still, creating new copies of the Arc from ArcBorrow::from_ref-created ArcBorrow was unsound, and that ability is the whole point of ArcBorrow, so yeah. As you can see, I went with a rename to from_ptr (instead of just changing from_ref to take a *const T argument) which should still be an easy to find method, it ensures users are becoming aware of the issue, and it documents the new safety requirements.