parser support for generator expressions so that we can give a better error message than "Expected ')'"

fix ticket #145 also for CondExprNode, make "bint (+) non-bint -> object" a general rule for independent spanning type finding, merge duplicated type determination functionality from CondExprNode back into PyrexTypes.py

code cleanup for if-const dead code removal: move handling into ConstantFolding transform, where it can also replace the entire node

--HG--
rename : tests/run/consts.pyx => tests/run/if_const.pyx