Structured activity annotations.

It could be very handy for users to support annotations nested in `NamesTuple`s/structs. Let's say we have

```julia
foo(a::Real, b::NamedTuple) = exp(log(a) + log(sum(b.x)) + log(b.y.q * b.y.r))

foo(4.2, (x = [4.0, 5.0], y = (q = 0.1, r = 0.2)))
```

and let's assume the user knows some parts of this are const. This currently doesn't work:

```julia
using Enzyme
a = Active(4.2)
b = (x = Const([4.0, 5.0]), y = (q = Active(0.1), r = Active(0.2)))
autodiff(foo, a, b)
```

```
ERROR: return type is Union{}, giving up.
```

But Enzyme is capable of handling nested mixed const/active/duplicated structures internally, of course. So the semantic equivalent

```julia
expanded_foo(a, b_x, b_y_q, b_y_r) = foo(a, (x = b_x, y = (q = b_y_q, r = b_y_r)))
da, d_y_q, d_y_r = autodiff(expanded_foo, a, b.x, b.y.q, b.y.r)
(a = da, y = (q = d_y_q, r = d_y_r))
```

works fine - it's just a bit hard on the user. :-)

Could `autodiff(foo, a, b)` be supported by just making some limited changes to Enzyme's "input formatting" stage?