BroadcastStyle of Differentials needs to match their primals

[oxinabox]

Particularlyatyers for Composite of subtypes of AbstractArray

For Thunks we need to unthunk at start, by overloading broadcasted

[oxinabox]

Yay, BroadcastStyle is a proper trait - it depends only on type.
And since the Composite differential is type-parametric on it’s primal type. Making them broadcast the same is trivial.
So Composite{<:AbstactArray} will work as well as a AbstractArray of differentials .
This is really pleasing.
This directly matters for #85

More generally this issue relates to #92 since operations between single deriviatives and chunked dervatives are broadcasting.

Also #96 is related in that it covers the cases not mentioned in the top of this thread.