forgot to use the loc...
Undoing most of the changes (and getting rid of the force-loc fields),
and refactoring into a s-hint-exp AST node type.  This will allow
various different hint types, which we can expand as needed or
desired.  For now, the only hint is h-use-loc, which lets us specify
the location to be used when reporting arity mismatches when calling a
function.