Improve routing to boost clock rate

[tdb-alcorn]

This change

• merges fixed point multiply and add into a single operation called mac, which avoids computing the saturation handling twice
• adds various input buffers to modules as needed to break up long routes

Ultra96v2 now makes timing at 150MHz with a WNS of 0.114ns. The longest routes now pass through the saturation handling in the acc ALUs.

Additional changes

• Re-use fixed-point MAC for times and plus;
• Golden MAC implementation to match hardware;
• Test fixed point with fuzzing;
• Fix fixed-point minus (discovered by fuzzing);
• Use ONNX ResNet-20 in simulation test;
• Use Mill fork args to allow simulation test with sufficient memory;