Rationalize assert support

* use pythran_assert instead of standard assert macro
* disabled when running under -DNDEBUG
Remove spurious parenthesis in expressions

They somehow disturb g++, which is a wonder to me...
PEP8 compliancy

I've been a bit lazy about these one lately... but not the only one :-)
Add support for the iter() intrinsic

Also rework the next() intrinsic so that it raises an exception when the
iteration stops. It implies a slight update of the generator stuff.
Fix false sharing in aliasing analysis
More rosetta code: Z letter is ok

Including a complex example that uses iter, next and a generator...