Let the user decide when a `&mut dyn self` is created

If callbacks are async and &mut dyn self is created before calling user function, all other functions depending on the same object will be blocked until this function is done

To solve this:

• Makes diff between async and non async functions
• If non-async: resolve before function call
• If async: don't resolve and provide a macro to create it

Or: Let the user create it anyway