auto merge of #15079 : nikomatsakis/rust/issue-5527-unify-refactor, r=pnkfelix

This is just a cleanup of the code. Doesn't really change anything deep about the way we operate. This is a prelude to implementing a good solution for one-way matching for #5527.

r? @pnkfelix (we were just crawling about this code, after all)

diff --cc src/librustc/middle/ty.rs
Simple merge

diff --cc src/librustc/middle/typeck/coherence.rs
Simple merge

diff --cc src/librustc/util/ppaux.rs
Simple merge