wip on Let

- move Let from api to core
- wip on bringing back LetScoper
- wip the triumphant return of LetScoper
  not so wippy it doesn't compile,
  but way too wippy to actually run and do anything.
- wip fixup
- Throwing out `LetVisitor`
  This was made redundant by our recent reintroduction of `LetScoper`
- update benchmark dumps for let changes
Adding a more-exhaustive test for ties
Making link prim breed names publicly accessible
Exposing breed names on `_linkbreedsingular`
Making breed names publicly accessible some more
upgrade to sbt 0.13.6

dodo, passenger pigeon, dinosaurs, the Confederacy, rotary telephones,
floppy disks, sbt 0.13.5.
upgrade to Scala 2.11.2 (all the way from 2.10!)
wip more Let work

- add comment
- wip FrontEnd cleanup (? questionable ?)
- wip more FrontEnd cleanup (? questionable ?)
- wip improve comments
- update Depend.scala to match current reality
- wip improve comments
- add a let test to FrontEndTests
  and clean up the corresponding code in LetScoper
- add a test case for bad task variable use
- wip delete obsolete comment