Implement instance table API

[zeux]

Similarly to having an option to bypass global device function pointers in favor of a device function table, we now provide an API to load instance pointers into a separate structure.

This makes it possible to create multiple instances and use them concurrently; this isn't needed as often as multi-device API but can still be valuable for some applications.

Fixes #137

(needs further testing)

[zeux]

This runs into a problem with vkGetDeviceProcAddr - it's an instance method so it's only loaded by volkLoadInstance, and without this volkLoadDeviceTable doesn't actually work because it relies on this global function. This will either require volkLoadInstanceTable to actually load this one global function, or we need to change volkLoadDeviceTable interface (or, well, add volkLoadDeviceInstanceTable or thereabouts that accepts both instance and device pointers).