Verify `UnwindSafe`ty for our types

[Lorak-mmk]

During pre-1.0 development we did not pay any attention to this trait. and unwinding in general.
Turns out we actually should, because it may cause semver-related issue - and fixing them by just implementing the trait blindly is not very elegant and defeats the purpose of this trait.
TODO before 2.0:

• Learn more about unwind safety
• Verify which of our types should implement UnwindSafe / RefUnwindSafe and make it so

Related: #1287 (comment)