Reflect on weird outparam corners

[Gankra]

TODO: what does it mean if you have multiple non-out-param outputs? Return a tuple? Error out on all known backends?

TODO: think through if the caller has any need to initialize an out-param (Currently irrelevant as we only suppport POD)

TODO: think through what it means to have an out-param of [&u32; 4]! (We should conservatively reject it for now.)