Constant Folding

Currently, we perform type inference on Decapodes to infer the degree and primality of differential forms.

However, we could also infer that computations derived solely from constants are also constants. I.e. Constant Folding is a special case of type inference on Variables.

So, we should leverage and augment existing type inference machinery to perform constant folding. The compilation of these down to simulation code is in the realm of Decapodes. The scope of a closing PR for DiagrammaticEquations is to just change uninferred nodes (and optionally Form-typed nodes) to constants when possible.

As with most PRs, this feature would be written differently if we stored whether a form is constant or not separately from the type of differential form it may be.