rustc: Add new graph library, based on code from region_infernece
rustc: Port region_inference to use the newly introduced graph library
auto merge of #6363 : nikomatsakis/rust/issue-6298-dataflow-graph-1, r=pcwalton

Extract the graph abstraction used in region inference and make it available as a general purpose module. First step towards establishing a common basis for forward and reverse dataflow.

r? @pcwalton