Aliased differentials and Inplace Accumulation

If a pullback is something like dx->(dx, dx) for dx being a reference type, e.g. like the pullback for +(::Array, ::Array), then the two outputs are the same object.
They are aliased.
Even though they are actually differentials for (usually) distinct (nonaliased) primal values.
This causes inplace accumulation to act wrong.

See discussion here https://github.com/FluxML/Zygote.jl/pull/962#issuecomment-835884201
where @mcabbott was just adding inplace accumulation for getindex.

I am wondering if we need to require that if the primals are not aliased the differentials also need to not be aliased.
This extra copies is slow if not inplace accumulation is not actually done though.
we might want to make it configurable depending on the AD system.
(this related to the config needs for #68)

I think the ideal solution would be copy-on-write objects.
But doing that without language support is suffering.

Start with the linked discussion in pull request #962, especially the getindex inplace-accumulation work and the relationship to configuration issue #68. Determine the intended aliasing semantics for differentials, whether copying should be configurable, and what behavior would demonstrate that inplace accumulation is correct.