Merge BuildReducedGraphVisitor into DefPathVisitor These two visitors run right after each other on the same immutable AST. There's also a hash map for transferring the TyCtxtFeed created in the def collector to the BRG when it visits the same items. There are possibly more avenues for sharing logic, but I want to keep this PR simple. only opening for perf runs for now. I'm still investigating how to ensure that future changes don't introduce subtle bugs by forgetting that def collection and reduced graph building are one pass now Best reviewed commit-by-commit. I took a lot of care for making the individual changes reviewable, but all the `Merge *` commits aren't able to compile libcore until the last one.