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.
Macros sometimes create auxiliary types and instances about them, and
they rely on the instance name generate to create unique names in that
case.
This modifies the automatic name generator to add a fresh macro scope to
the generated name if any of the constants in the type of the instance
themselves have macro scopes.
Closes#2044
