catch C code error of unset temp results with an assert at code generation time

--HG--
extra : amend_source : a1b27560de97695dde4b9fea41a39f7f10de736c
remove duplicate code
be more verbose in compiler crash case when temp result name is missing
clean up some whitespace
fix legitimate test for missing result temp name
when replacing SimpleCallNode with PyMethodCallNode, make sure we replace all cloned references to the node, not only the current one
simplify WithTargetAssignmentStatNode and make it more robust against replacements of the context manager node; undo node.result() checking as it broke TempNode's disposal code