[DISCUSSION] Split off blocked tuples/permutations into separate package

[mtfishman]

Consider splitting off blocked tuples/permutations into a separate package called BlockTuples.jl (analogous to BlockArrays.jl but for tuples), since that functionality is being used by both TensorAlgebra.jl and FusionTensors.jl. Though as discussed we'll hold off on that for now to try to keep the package structure simpler. That would allow turning TensorAlgebra.jl into a weak dependency of FusionTensors.jl through a FusionTensorsTensorAlgebraExt package extension.