lowering TypedArray/Array#at method

[Gus Caplan]

Hi all,

I'm working on adding reductions for the recently added relative indexing methods (String#at, Array#at, TypedArray#at). I've created a lowering for String#at successfully but I'm not sure what to do for Array#at and TypedArray#at. If I understand correctly, they need to handle all the different ElementKinds and ExternalArrayTypes and then emit loads to the underlying fixedarray or buffer based on that, but I'm not quite sure how to go about that. I'd also like to provide a fast path for `Array.prototype.at.call(array-like, index)`, but I'm not sure how to emit the generic property loads from the js call reducer.

Thanks