Add some more commentary to FFI tutorial.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
Check the privacy of implemented traits

This bug showed up because the visitor only visited the path of the implemented
trait via walk_path (with no corresponding visit_path function). I have modified
the visitor to use visit_path (which is now overridable), and the privacy
visitor overrides this function and now properly checks for the privacy of all
paths.

Closes #10857
auto merge of #10862 : alexcrichton/rust/issue-10857, r=huonw

This bug showed up because the visitor only visited the path of the implemented
trait via walk_path (with no corresponding visit_path function). I have modified
the visitor to use visit_path (which is now overridable), and the privacy
visitor overrides this function and now properly checks for the privacy of all
paths.

Closes #10857
auto merge of #10822 : ezyang/rust/master, r=huonw

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>