Loc.Error now carries a diagnostic: a stable kind, a span, notes that each
have their own span and severity, and the macro expansion it came from. The
notes are the part that was actually missing — "this is wrong here" plus
"because of that, over there" is two places and two explanations, and a
single string can state only one of them.
The compatibility story for the daemon, which was the open question: the
single-diagnostic exception stays the single-diagnostic exception. Session
and dev evaluate one form and have one failure to report, so they take a
location and a message out of it with Loc.summary and are otherwise
unchanged. A second exception carries a list, and only a driver that
compiles a whole file raises it, so nothing interactive has to know it is
there.
No message text changed.
Loc.t grows an exclusive end, defaulting to the start, so a location nobody
widened is a zero-width span at a point and every existing call site keeps
its old meaning. Only the reader knows where a form ends, so only the reader
fills them in — one helper in the one place that holds both ends, which is
why nothing above Reader had to learn a span exists.
The width assertion is the point of the tests: the field could exist, nothing
could fill it, and every underline would be one character long while the
feature looked finished.
