Implement support for new representation of spatial derivatives of functions after UFL has applied differentiation algorithm

UFL has since before the summer had an alternative way to represent spatial derivatives of functions after the differentiation algorithm has been applied. This cannot be enabled by default in UFL because it will break existing form compilers. Supporting this in FFC will probably improve scalability for complicated forms right away, and is a prerequisite for the work-in-progress algorithms from uflacs to work properly.

The issue is the representation of spatial derivatives of functions (Coefficient/Argument) after the differentiation algorithm has
been applied. In the current vs new implementation, they are represented as e.g.

SpatialDerivative(Coefficient(V), MultiIndex((i,)))
vs
Indexed(Grad(Coefficient(V)), MultiIndex((i,)))

or

ComponentTensor(SpatialDerivative(Coefficient(V), MultiIndex((i,))), MultiIndex((i,)))
vs
Grad(Coefficient(V))

So basically, the difference is that you will not encounter SpatialDerivative in an expression after expand_derivatives has been applied, but you will encounter Grad. Other derivatives like div and curl are represented in terms of Grad instead of SpatialDerivative.

The important property of the new representation is that indexing operations are distinct from operations introducing new actual values into an expression, which makes it a lot easier to identify common subexpressions involving Grad(f).

To enable this behaviour change in ufl, you need to do this before anything else:
    import ufl.algorithms.ad
    ufl.algorithms.ad.expand_derivatives = ufl.algorithms.ad.expand_derivatives2

I don't feel up to the task of implementing this without risking breaking lots of stuff, but I'll be happy to review changes and answer any further questions!