It's just a wrapper around hashtab, but it makes checking if a string is
in a set easy. Way overkill when only a few extensions are enabled, but
more might come later.
This paves the way for clean initialization of the Vulkan renderer, and
very much cleans up the older renderer initialization code as gl and sw
are no longer intertwined.
A single graphics-capable queue should be enough for now. However, I'm
not sure I'm happy with a lot of the code: it's a bit difficult to write
flexibly configured code for Vulkan (or seems to be at this stage),
especially in C.
