Add mechanisms to make builder API more convenient

[hannobraun]

Add ObjectArgument a trait that abstracts over objects passed as arguments to builder API methods. Check out the commits for details, but the gist of it is, with the help of this trait, builder methods can now be used like this:

// Pass an arbitrary number of points, get an arbitrary number of edges back.
// Both argument and return values are `Vec`s of the same length.
let edges = cycle.add_edges_from_points(points);

// Pass a specific number of points, and get the same number of edges back.
let [edge_a, edge_b, edge_c] =
    cycle.add_edges_from_points([point_a, point_b, point_c]);

This makes situation in which you are working with a specific number of objects much more easy to deal with.

This pull request adds the infrastructure to do this and applies it to one builder method, simplifying a use of that builder method significantly. I expect to use this heavily going forward, as I work towards #1162.