Type error messages have typedefs expanded away

[catamorphism]

These are all in compile-fail. Presumably due to @pcwalton 's recent commits.

[catamorphism]

Not critical for 0.6, de-milestoning

[catamorphism]

unsafe-fn-autoderef was easy to update -- I filed #6482 . The other two still fail because type error messages print types with synonyms expanded away. I'm not sure how important it is to preserve type synonyms in error messages. It can certainly make error messages more readable sometimes.

I guess I'll nominate for milestone 5, production-ready, so we can discuss it; also will change the bug title to something more descriptive.

[graydon]

just a bug, removing milestone/nomination.

[catamorphism]

Visiting for triage. Still important.

[catamorphism]

I can't figure out which test cases were affected here, and grepping for "4374" in the repo doesn't turn up anything, so I'm closing this.