What happens here is that we lower `: ` to a missing expression, and
then correctly record that the corresponding field expression resolves
to a specific field. Where we fail is in the mapping of syntax to this
missing expression. Doing it via `ast_field.expr()` fails, as that
expression is `None`. Instead, we go in the opposite direcition and ask
each lowered field about its source.
This works, but has wrong complexity `O(N)` and, really, the
implementation is just too complex. We need some better management of
data here.
Conjecture: it's impossible to use hir::Path *correctly* from an IDE.
I am not entirely sure about this, and we might need to add it back at
some point, but I have to arguments that convince me that we probably
won't:
* `hir::Path` has to know about hygiene, which an IDE can't set up
properly.
* `hir::Path` lacks identity, but you actually have to know identity
to resolve it correctly
