Fix unwrapping exceptions from decorated functions

[mwaskom]

Fixes an issue introduced in #194 where synchronicity would raise its internal UserCodeException class when exceptions were triggered within:

• async functions
• wrapped with a decorator
• invoked via the .aio interface

The correct behavior would be to unwrap the UserCodeException and propagate the underlying exception.

[mwaskom]

Confirmed the only other direct uses of inspect.iscoroutinefunction are in the unit tests

[erikbern]

great to have a test!

[freider]

Nice catch! Side note: I think we should be able to get rid of wraps_by_interface - it sort of served the same purpose as is_coroutine_function_follow_wrapped.

The added benefit of being able to inspect.iscoroutinefunction on the wrapped function feels a bit unnecessary if we aren't doing it ourselves anyways