Generate packed arrays via VAST codegen

Currently we use unpacked arrays, which end up generating lots of unrolled assignments for propagating the array values around. Mark is working on switching us to packed arrays.

I believe https://github.com/google/xls/commit/e69f82f3299a9ab6e79f7f108fc3d8096fdb3c73 did a bunch of it but there was something remaining to do finish this up, Mark can you remind me what that was?