Fix compilation of none types by clang

Currently, none type only overloads operator +/*- for operands of the
same type, thus adding an `int' and a `none<long>' leads to overloading
ambiguity.
Fix a few memory leaks

Leaks during GMP <> long conversion, a reference decrement was missing.
Fix bug when meeting a function that always raises

A function that always raises an exception does not have a return None
inserted and thus had no return_type.
Merge pull request #141 from serge-sans-paille/always-raising

Always raising