Change CompArrow to Circuit

[zenna]

There is a conflation of concepts, I have:

Arrows which are like Functions PrimArrows which are like primitive, built-in functions, or IntrinsicFunctions CompArrows which are like Functions

I think instead, a what is currently a CompArrow is really about the program graph, a computational graph. Instead I should have

"A computational graph"
mutable struct Circuit
  edges::LG.DiGraph
  ....
end