Have something like `forward.chunk` for `ForwardDiff.Chunk`?

[Non-Contradiction]

If users want to use a chunk of certain size, they need to do it like this:

c = 10
JuliaCall::julia_assign("c", c)
JuliaCall::julia_eval("ForwardDiff.Chunk{c}()")

which is quite inconvenient.

Although it's not in the basic API of ForwardDiff, it's in the advantage usage guide and is recommended "If your input dimension is constant across calls, you should explicitly select a chunk size rather than relying on ForwardDiff's heuristic. "

So my idea is that we should wrap the related Chunk functions.

[Non-Contradiction]

After more thinking, a chunk_size argument can be added to the Config methods. A separate forward.chunk method may be too much with setting chunk size as the only functionality. And it looks more idiomatic from R's point of view.

[Non-Contradiction]

And the new chunk_size parameter can replace the chunk parameter we currently have.

[Non-Contradiction]

The change in the API has been finished, documentation and testing has been updated.