Introduce attributes to store information about reduction dimensions (#224)

* Rename EinsumAxisDescriptor subclasses.

In order to avoid coming off as something that is more generally applicable to
any pt.Array node.

* adds helpers to get all reduction induction variables

* implements ReductionDescriptor

* defines IndexLambda.var_to_reduction_descr

* defines Einsum.redn_axis_to_redn_descr

* test reduction_axis

* Propagates loopy iname tags for ReductionAxis

* tests loopy tag propagation for ReductionAxis

* introduces ReductionAxis/EinsumIndex specific exception types

* prefer immutables.Map over pyrsistent.PMap