Typed Racket history for 6.0.1.

Please include in release.
Fix an unfortunate bug in Union construction.
Improve TR timing, add more timing events.
Misc minor improvements.
Initial work on refactoring inference not to use exceptions.
Fix stale contract.
Weaken failing contract. Not a real fix.
fixup tests
Address most comments, fix some dumb bugs.
All unit tests pass.
Build passes.
Better printing for debugf