This PR updates the formatting of, and adds explanations for, "unknown
identifier" errors as well as "failed to infer type" errors for binders
and definitions.
It attempts to ameliorate some of the confusion encountered in #1592 by
modifying the wording of the "header is elaborated before body is
processed" note and adding further discussion and examples of this
behavior in the corresponding error explanation.
This PR changes how the unfold theorems for well-founded recursion are
created. They are created eagerly (anticipating that the behaivor may be
affected by simp sets soon), and without the detour of going through
equational theorems.
