Describe cl-vk's conditions for misuse and its escape hatches
Misuse is a type-error where check-type can say what is wrong, and a
vk-error with a message otherwise; the struct macros' expansion-time
errors are vk-errors too. instance-commands, device-commands and
swapchain-images are escape hatches that refuse destroyed objects, and
swapchain-images has an entry of its own. device-queue takes :family
and :index. with-structs takes a foreign pointer for a field that
points to untyped memory. The generator no longer has *platforms*.
