[WIP] [BlockSparseArrays] Change behavior of non-blocked slicing

[mtfishman]

Addresses one of the issues being tracked in ITensor/BlockSparseArrays.jl#2, based on a design discussed in JuliaArrays/BlockArrays.jl#347.

To-do:

• Fix printing blocked views, for example show(stdout, MIME("text/plain"), @view(a[[Block(2), Block(1)], [Block(2), Block(1)]])), and add a test.
• Raise an issue in BlockArrays about x = randn(2, 2); blocks(x)[1, 1] not returning x, but instead returning a view object which is more challenging to work with (Make blocks(randn(2, 2))[1, 1] return the original array JuliaArrays/BlockArrays.jl#403).
• Get tests passing.
• Add new tests.

[mtfishman]

I've decided to go with a different design, closing in favor of #1486.