Add Rewrite Rule construction from a peak simulation function (which uses multiple PEs)

[rdaly525]

It would be great to specify something like the following as a Rewrite rule (or library function)

def FMA(a : Data, b : Data, c : Data):
   x = PE(Inst.Mul,a,b)
   y = PE(Inst.Add,x,c)
   return y

[rdaly525]

@leonardt, I think this should be possible using your RTL generation capabilities? I would want the output to be an FMA module that instantiates 2 PE's configured appropriately (Inst.Mul, and Inst.Add).

Do you think something like this is possible?