Make NamedTuple(::ComponentArray) more AD friendly

[marius311]

Here's a version of this which doesn't mutate an array, hence works with Zygote:

function _namedtuple(x::ComponentVector)
    NamedTuple{keys(x)}(map(valkeys(x)) do key
        _namedtuple(getproperty(x, key))
    end)
end

And a test which otherwise fails with the latest release version:

@test ComponentArray(x=4,) == Zygote.gradient(ComponentArray(x=2,)) do c
    (;c...,).x^2
end[1]

Wanted to just suggest this here rather than PR since there's some stuff that function is doing (or seemed to before comments) that I wasn't sure about. If this is good, please feel free to commit directly.

[jonniedie]

Oh, this is great! Really speeds things up too.

[jonniedie]

Thanks!