Generalize item_span into node_span, which works on more types.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
auto merge of #11070 : ezyang/rust/better-errors, r=alexcrichton

On the advice of @huonw, I've just replaced item_span outright.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>