Simplify indexed ListTensor objects

[pbrubeck]

When splitting mixed Forms, mixed Arguments get replaced by ListTensors of Zero and subspace Arguments of the form [0, ..., 0, v[0], ..., v[k], 0, ..., 0]. The resulting split Form would contain many instances of Indexed objects with this ListTensor, that could be simplified to either 0 or [v[0], ..., v[k]] -> v.

Previously, the simplification step was left as a task for the form compiler, but working with such complicated expressions often results in sub-optimal code generation. This PR addresses the following simplications at the UFL-level:

1. Indexed(ListTensor[v0, ..., vk], (j,)) -> vj
2. ListTensor(Indexed(v, FixedIndex(i)) for i in range(k)) -> v
3. Restricted(ConstantValue) -> ConstantValue

As a consequence, splitting a bilinear Form with a block in the diagonal equal to zero produces an empty Form, losing all the information that the unsimplified Form carried around, such as the Arguments and the arity. This situation needs to be handled carefully as a special case. See firedrakeproject/firedrake#3947

With these simplifications, we can correctly estimate quadrature degrees for integrands that only depend on one component of the solution, thus lowering flop counts.

[jhale]

This seems fine to me and thanks for the various modernisations added as well.