Linear solvers

[ChrisRackauckas]

A_ldiv_B! and \ support for GPUArrays is pretty essential and I keep assuming it already exists.

[simsurace]

Would it be appropriate to add some overloads for Diagonal{AbstractGPUArray} analogous to those for cholesky I added here?

[maleadt]

Can you implement them using GPUArrays' abstractions, or a GPUArrays kernel? If so, then yes this would be a good fit here.

[simsurace]

Actually, for diagonal matrices we can just use broadcasting syntax. I'll open a PR.

[kshyatt]

I think we do have ldiv! support now, feel free to reopen if there's more you need