FillArrays

[willtebbutt]

There are currently instances when we want to use arrays of zeros in which size information is retained - this is not something supported by the Zero differential, but which is supported by FillArrays.jl's Zeros type. I would be nice to use this here, but are there perhaps issues with it not defining setindex?

Related to #15

[willtebbutt]

FWIW, I'm very keen on having first-class AD support for FillArrays as I use them extensively in Stheno.jl, and Zygote.jl now makes use of them internally rather than rolling its own fill type.

[oxinabox]

tr is an example that should be FIllArrays.
Zygote does so.