A cost model for Nim v2

[Araq]

* `except Exception` is a thing. Sure the hierarchy is usually not helpful but 
it can be and it doesn't hurt.
* So preallocate the exception. It's not a big deal and embedded systems can 
come with a heap anyway, it's not 1970 anymore.
* So add this special handling. It's about 10 lines for Malebolgia and the 
benefit is that people then can use exceptions.