Add as_boxed method to MutableArray

[sd2k]

MutableArray has as_arc, but not as_boxed (or into_boxed? not sure which is more appropriate) right now, although GrowableArray does - is this something you would consider adding? I have a use case which requires it to implement Deserialize for a Box<dyn Array> wrapper 🙂

Happy to send in a PR if this sounds good. Thanks!

[jorgecarleitao]

Sounds great! Yes, in general it is good to have both methods. Rust does not have an idiomatic way of writing a function that can return either, so we need to do it ourselves 👍

Thanks for volunteering for working on it; let me know if you get stuck :)