Don't mark fill! non-differentiable

[mcabbott]

Closes #514:

julia> Zygote.gradient((x,y) -> sum(fill!(x,y)), [1,2,3], 4)
ERROR: Mutating arrays is not supported -- called setindex!(::Vector{Int64}, _...)

Original non-diff rule being removed here is from #310 I believe.

[CarloLucibello]

This caused regressions in some of my code. In fact, it is not uncommon to write a = fill!(similar(x), 0),
we have many examples also in this repo. I'll file a PR with the rrule!

[mcabbott]

Sorry about breaking things.

But how can that work without getting this wrong? Sorry, this is the bit that's easy to get right:

julia> Zygote.gradient((x,y) -> sum(fill!(x,y)), [1,2,3], 4)
(nothing, nothing)

julia> Zygote.gradient((x,y) -> sum(map(_ -> y, x)), [1,2,3], 4)
([nothing, nothing, nothing], 3)

The hard part is that similar(x) might be used elsewhere, and I don't think Zygote can know.