Investigate Nengo graph rewrite optimizations

[arvoelke]

The final spiking matrix inversion example currently takes ~27 seconds with Nengo optimize=True, ~52 seconds with Nengo optimize=False, and ~32-38 seconds with NengoDL. The model is really just 500 recurrently connected neurons being simulated for 2.5 seconds (2,500 time-steps), although it is being built up using lots of passthrough nodes and a separate ensemble for each neuron. This is likely causing issues for both Nengo and TensorFlow that could be solved by some larger model rewrite optimizations in Gyrus before handing off the model to the backend, or by extending the Nengo core optimizer.

More context: https://forum.nengo.ai/t/gyrus-running-time/1584/5