vulkan: load all function pointers indirectly

We already had almost all of the infrastructure for this anyway, and there were legitimate use cases that benefit from this change. I decided to drop the extra vk prefix from our function pointers, because writing VK(vk->vk seemed a bit silly and doesn't provide any real advantages (tab completion etc. still work on the reduced function pointers, and development of new code can be done without loading the function pointers anyway).

Closes #75 (closed).