This commit includes richer projections for both Places and
UserTypeProjections. However, the tests only touch on Places. There are
also outstanding TODOs regarding how projections should be resolved to
produce Place types, and regarding if UserTypeProjections should just
contain ProjectionElem<(),()> objects as in MIR.

Asserting is_empty is slightly more concise.

It's not clear to me (klinvill) that UserTypeProjections are produced
anymore with the removal of type ascriptions as per
rust-lang/rfcs#3307. Furthermore, it's not clear
to me which variants of ProjectionElem could appear in such projections.
For these reasons, I'm reverting projections in UserTypeProjections to
simple strings until I can get more clarity on UserTypeProjections.

Also shifts comments explaining why Stable MIR drops an optional variant
name field, for `Downcast` projection elements, to the `Place::stable`
function.