vulkan.h: Change return types of some functions to void

Functions that should be thought of as "impossible to fail in the face
of valid parameters" have had their return types changed to void.
This includes all of the vkDestroy functions, vkFreeMemory, and vkUnmapMemory.
vkUpdateDescriptorSets is also included, because of the frequency the function
is expected to be called.
21 files changed