Tweak API for using a custom storage in graphs

[pnevyk]

Currently, the way to specify a custom storage when initializing a graph is this:

let mut graph = Graph::with_storage(AdjMatrix::<_, _, Undirected, DefaultIndexing>::new());

The need for specifying the Undirected and DefaultIndexing generic parameters brings unnecessary clutter. Moreover, we should follow the convention of the Rust standard library, which uses new_in naming convention for specifying allocators.

To improve the user experience, we should make these changes:

• Provide undirected and directed constructor methods, the same way as we do with default new_undirected and new_directed. This removes the need of specifying directionality generic parameter.
• Use new_in naming convention. That is, there will be these three constructors: new_in (directionality unspecified), new_undirected_in and new_directed_in.
• For the graph storages, change the implementation of Default to fix the indexing to DefaultIndexing. This removes the need of specifying DefaultIndexing (but a custom indexing would still be allowed via new (e.g., AdjList::new())).

After all, the above example will look like this:

let mut graph = Graph::new_undirected_in(AdjMatrix::default());