and update fact generation to the new relation names

- "polonius_fact_generation" is dedicated to profiling the Polonius fact generation, from the MIR and NLL constraints
- "polonius_analysis" is dedicated to profiling the duration of the Polonius computations themselves: move/init analysis, liveness, borrowck-ing

This prepares the code base for when projection is interned. Place's
projection field is going to be `&List<PlaceElem<'tcx>>` so we won't be
able to pattern match against it.

The commit should have changed comments as well.
At the time of writting, it passes the tidy and check tool.

Revisions asked by eddyb :
- Renamed of all the occurences of {visit/super}_mir
- Renamed test structures `CachedMir` to `Cached`

Fixing the missing import on `AggregateKind`

We  equate the type in the annotation with the inferred type first so
that we have a fully inferred type to perform the well-formedness check
on.

Added `mir::UserTypeProjection`, a stub for a structure that projects *into* a given UserTypeAnnotation.

(That is, it will pull out some component type held or referenced by
the type annotation.)

Note: this still needs to actually do projection itself. That comes in
a later commit