Optimised Wrappers

This pull request uses the optimised wrapping mechanism available from https://github.com/ImageEngine/cortex/pull/239. With this and #771 I'm seeing almost a 20% reduction in time to compute shader networks, and even more substantial gains generating shaders in a threaded context. We're not entering Python at all now during these tasks, so one big threading efficiency hurdle is out of the way.

Only one commit is used to actually move to the new style of binding. The others are all correcting assumptions about object identity and lifetime that no longer hold true, and fixing latent bugs which were exposed by changes in the lifetime of ScriptNode objects since they no longer rely on garbage collection. Be aware...