Add path suffix lookup

Extended `ast_map::Map` with an iterator over all node id's that match a path suffix.

Extended pretty printer to let users choose particular items to pretty print, either by indicating an integer node-id, or by providing a path suffix.

 * Example 1: the suffix `typeck::check::check_struct` matches the item with the path `rustc::middle::typeck::check::check_struct` when compiling the `rustc` crate.

 * Example 2: the suffix `and` matches `core::option::Option::and` and `core::result::Result::and` when compiling the `core` crate.

Refactored `pprust` slightly to support the pretty printer changes.

(See individual commits for more description.)

I don't think `extern { ... }` counts as a mod for this purpose? (It doesn't have a meaningful name, in any case.)