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.)