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- SILPackType carries whether the elements are stored directly in the pack, which we're not currently using in the lowering, but it's probably something we'll want in the final ABI. Having this also makes it clear that we're doing the right thing with substitution and element lowering. I also toyed with making this a scalar type, which made it necessary in various places, although eventually I pulled back to the design where we always use packs as addresses. - Pack boundaries are a core ABI concept, so the lowering has to wrap parameter pack expansions up as packs. There are huge unimplemented holes here where the abstraction pattern will need to tell us how many elements to gather into the pack, but a naive approach is good enough to get things off the ground. - Pack conventions are related to the existing parameter and result conventions, but they're different on enough grounds that they deserve to be separated.