Instead of walking over the source decls, and looking up the Name
to find the TyCon or whatever, we just walk over the list of
TyThings that have been brought into scope.  This is much tidier.

The only wrinkle is that, since we don't have the original declaration,
we don't have its SrcSpan to put in the error message.  I fixed this
by making the SrcSpan for the TyCon itself be the span of the whole
declaration.  This actually makes sense anyway.

There are bunch of error message wibbles in consequence.