StructArray convenience methods (redux)

[hohav]

I see #61 is marked as completed, but the methods discussed don't seem to exist. Besides that, I'd guess that most of the time when a caller knows a certain field exists, they probably also know its type. So I'd love to be able to do something like this:

arr.child::<PrimitiveArray<u64>>("id").unwrap()

Thoughts? I can take a stab at a PR, if it seems worthwhile.