Draft: Allow unsafeDupableInterleaveIO to inline

While trying to learn more about our black-hole mechanism(s), I found the reasoning in Note [unsafeDupableInterleaveIO should not be inlined] impossible to follow. So I replaced it.

Also, it doesn't make much sense to hurt uses of the more-unsafe dupable versions of these operations with NOINLINE when really the problem is ensuring the non-duping versions really don't allow duplication.

This changes base, so it has a CLC proposal.