Add method of calling instance-level commands from device extensions without creating a device first

[KyleMayes]

https://github.com/KyleMayes/vulkanalia/issues/281 raised the issue of some device extension commands actually being instance-level commands.

These are no longer loaded with vkGetDeviceProcAddr as part of the fix for that issue, but this still leaves no way (without dropping down into loading the functions yourself) to call these commands before creating a device.