Improve BlockUnitRange indexing #350

Job	Run time
IntegrationTest (ITensorBase) / ITensorBase	1m 40s
IntegrationTest (NamedDimsArrays) / NamedDimsArrays	1m 40s
IntegrationTest (GradedArrays) / GradedArrays	1m 35s
IntegrationTest (QuantumOperatorDefinitions) / QuantumOperatorDefinitions	1m 27s
IntegrationTest (KroneckerArrays) / KroneckerArrays	1m 23s
IntegrationTest (FusionTensors) / FusionTensors	1m 36s
	9m 21s

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