remove box data from shape tables #1739
Description
Currently, the shape table fully describes the contents of all boxes. However, boxes now uniformly carry a type descriptor of their contents. Therefore, we should be able to replace boxes in shape tables with a single byte ("box"), and then load the shape table for the box contents from the box itself. This will be helpful for monomorphization (#1736) as well, because it allows a single version of the function to operate over all box types without derived type descriptors.