Allow trailing , in macro arguments at 0.13.0-alpha.3

[MOZGIII]

This is rejected:

counter!("processing_errors", 1,
    "component_kind" => "transform",
    "component_type" => "add_fields",
);

While this works:

counter!("processing_errors", 1,
    "component_kind" => "transform",
    "component_type" => "add_fields"
);

Used to work before at 0.12.

[tobz]

I think we can close this now with the latest alphas having this.