Reduce header inclusions of index_set.h in evaluation_kernels

[kronbichler]

This PR removes the need to include index_set.h and partitioner.h from the expensive compute files matrix_free/evaluation_kernels.h. The untangle this dependency, I had to avoid including index_set.h from mpi.h (because aligned_vector.h necessarily includes mpi.h, and we need AlignedVector), for which I added a forward declaration and a few similar types IndexSet::size_type -> types::global_dof_index. The former is defined as the latter,

dealii/include/deal.II/base/index_set.h

Lines 76 to 80 in 706a51c

	/**
	* @p size_type is the type used for storing the size and the individual
	* entries in the IndexSet.
	*/
	using size_type = types::global_dof_index;

so the change is equivalent. To further strengthen the link and avoid divergence in the two code paths, I added some static asserts.