[NDTensors] Introduce NestedPermutedDimsArrays submodule

[mtfishman]

This introduces a NestedPermutedDimsArrays module that defines a NestedPermutedDimsArray type, which is like Base.PermutedDimsArray but is singly nested, i.e. it is a lazily permuted array where the elements are also lazily permuted.

This is meant to serve as the new output type of blocks(::PermutedDimsArray) in BlockSparseArrays, i.e. blocks(a::PermutedDimsArray) = NestedPermutedDimsArray(blocks(parent(a)), perm(a)), as an alternative to the current design based on SparsePermutedDimsArrayBlocks. That will make the PermutedDimsArray wrapper more in line with the design of the Adjoint and Transpose wrappers as of #1580. However, in order for that to work, we will also have to add support for NestedPermutedDimsArrays in SparseArrayInterface, which will be handled in a future PR.

@lkdvos

Note that something similar could be achieved by composing a PermutedDimsArray with a MappedArray where the map (and inverse map) itself constructs a PermutedDimsArray, and in fact that was an earlier design I used in BlockSparseArrays. Maybe we can consider that in the future, though dealing with nested wrappers might be a bit tricky to deal with in SparseArrayInterface right now, so this approach feels simpler for now.