Add Send + Sync bound to load result.

[bjadamson]

Hello! Thank you for making this crate, I have been using it to make a game for the past several months.

I integrated the anyhow crate into my project, and after reading this thread I learned I needed to convert the Box returned by easy_gltf::load by adding the Sync + Send bounds to allow the conversion to happen.

With this change I am now able to convert the Box<dyn std::error::Error> returned from easy_gltf::load into a anyhow::Result<T, E>

let scenes = easy_gltf::load(path).map_err(|e| anyhow!(e))?;

Without this change I am unable to figure out another way. I've only been using rust for a few months now, so it's totally possible I don't understand something. Thanks