Make *Node Great Again

[chewxy]

The earliest design had *Node implement tensor.Tensor. Time to make that happen again. h/t @docmerlin for suggesting that

So the general gist is this:

1. Create a Tensor interface that mirrors a subset of tensor.Tensor (without the private methods).
2. *Node implements Tensor
3. Create a SymbolicEngine method that implements tensor.Engine
4. Write generator to generate methods in operations.go and other operations. The methods will change from: func Add(a, b *Node) (*Node, error) to func Add(a, b Tensor) (Tensor, error) . This allows for passing in tensor.Tensor in those, which will make the API a LOT simpler.
5. Simplify the data structure for *Node to remove pointers to prevent GC from chasing them
6. Share tensor.Shape object between tensor.Tensor and *Node #163