WIP: Add AutoBorrowTraitObj AutoRefKind, and start implementing relevant adjustments

This partially implements (1) and (3) from my IRC conversation with
Niko:

 (1) add a new kind of adjustment for objects
 (3) modify the method code to borrow in search_for_autosliced_method()

I also added FIXMEs to things I wasn't sure about. Remove them if the
behaviour is correct, or fixed in the future.

cc #5762
WIP: Implement auto-borrow adjustment for trait objects in trans/expr.rs

cc #5762 (incomplete fix)
WIP: Ensure that @self methods are only called on @Trait obs, etc

cc #5762