Emit block members directly in the IO structs and sort them.
Ensures we can get some kind of stable order between stages.
To complete the story, we'll need to be able to inject unused inputs /
builtins, or eliminate unused outputs (probably easiest solution).
MSL does not support value semantics for arrays (sigh), so we need to
force constant references and deal with copies if we have a different
address space than what we end up guessing.
