[Cython] nonecheck directive

[Stefan Behnel]

Hi,

why is the "nonecheck" directive set to False by default? Shouldn't it 
rather be a "I know what I'm doing" option that allows advanced users to 
trade speed for safety?

The reason I'm asking is that I just enabled its evaluation in 
NoneCheckNode and immediately got crashes in the test suite. So its 
currently only half-heartedly safe because it's not being evaluated in a 
lot of places. That's a rather fragile situation, not only for refactorings.

Stefan