LinearAlgebra: use band index in structured broadcast

[jishnub]

This adds a new BandIndex type internal to LinearAlgebra that parallels a CartesianIndex, and stores a band index and a linear index of an element along that band. If the index of the band is a compile-time constant (which is often the case with Diagonal/Bidiagonal/Tridiagonal matrices), constant-propagation may eliminate branches in indexing into the matrix, and directly forward the indexing to the corresponding diagonal. This is particularly important in broadcasting for these matrices, which acts band-wise.

An example of an improvement in performance with this PR:

julia> using LinearAlgebra

julia> T = Tridiagonal(rand(893999), rand(894000), rand(893999)); # a large matrix

julia> @btime $T .+ $T;
  5.387 ms (10 allocations: 20.46 MiB) # v"1.12.0-DEV.337"
  2.872 ms (10 allocations: 20.46 MiB) # This PR

julia> @btime $T + $T; # reference
  2.885 ms (10 allocations: 20.46 MiB)

This makes the broadcast operation as fast as the sum, where the latter adds the diagonals directly.

I'm not 100% certain why this works as well as it does, as the constant band index may get lost in the newindex computation. I suspect branch prediction somehow works around this and preserves the constant.

[jishnub]

Gentle bump