ast_map: Added iterator over all node id's that match a path suffix.

ast_map: Added iterator over all node id's that match a path suffix.

This is useful e.g. for tools need a node-id, such as the flowgraph
pretty printer, since it can avoids the need to first pretty-print the
whole expanded,identified input in order to find out what the node-id
actually is.

It currently only supports path suffixes thst are made up of module
names (e.g. you cannot use the type instantiation form `a::<int>::b`
or `option::Option::unwrap_or` as a path suffix for this tool, though
the tool will produce paths that have non-modulues in the portion of
the path that is not included in the suffix).

(addressed review feedback too)