Canada needs to invest an estimated $200 billion in public infrastructure to help keep private sector industry competitive, according to a new study by the Institute for Research on Public Policy.   In the report, University of Waterloo economist James Brox analyzed the condition of Canada's infrastructure and its impact on productivity, specifically within the manufacturing sector.   The study argues that Canada's competitiveness depends on a modern, efficient and well-maintained public infrastructure network, and that productivity in Canada's manufacturing sector h