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.
Instead of the previous constraints on the right hand side that only
allowed a permutation of variables as parameters to a function the
new heuristic allows anything to the right of a function as long as
each variable only appears at most once.
