zipped iteration should splat

Currently we have
```julia
StructWalk.scan(model, grad) do tupl
x, g = tupl
# ...
end
```
while I think it would be more idiomatic to pass separate arguments instead of a tuple
```julia
StructWalk.scan(model, grad) do x, g
# ...
end
```
Of course this would be a breaking change, but it would match the semantics of `map` etc...

Start by locating the StructWalk.scan entry point and inspect how its callback currently receives the tuple shown in the issue. Change the callback behavior so zipped iteration passes x and g as separate arguments, then verify the existing scan usage and any related checks reflect the new calling convention.

Written by the indexing model from the issue text.