This commit modifies the UserAssertTy statement to take a canonicalized
type rather than a regular type so that we can handle the case where the
user provided type contains a inference variable.

Improvements:

- Use Clone not Copy for the "simple cases"
- Separate TypeFoldable and Lift for the "simple cases"
- Support generics type parameters
- Support named fields in enum variants
- etc

It was too easy to get this wrong

Added `-Z two-phase-beyond-autoref` to bring back old behavior (mainly
to allow demonstration of desugared examples).

Updated tests to use aforementioned flag when necessary. (But in each
case where I added the flag, I made sure to also include a revision
without the flag so that one can readily see what the actual behavior
we expect is for the initial deployment of NLL.)