Bug in Flows: loops are not supported, but repeated handlers cause loops

See example

asya/examples/flows/if_nested.py

Line 14 in b3b9839

p = handler_b(p)

it compiles into https://github.com/deliveryhero/asya/blob/main/examples/flows/compiled/if_nested/flow.png

Now asya flow compiler should explicitly disallow such loops and print at least big fat warning