Progress of Wrapping APIs of ForwardDiff: Part 2

[Non-Contradiction]

This is as a continuation for #2. Since the recent progress and some new thinking, open another issue to track the progress.

Now we have achieved:

• Non-mutation-related API of ForwardDiff.jl has been fully wrapped in a quite R idiomatic way. Ordinary users can have access to all functionalities provided by non-mutation-related API of ForwardDiff without any hacking or using JuliaCall explicitly.
• Basic documentation is in place.
• Tests on these implementations have been in place, corresponding to testing on the derivative, gradient, hessian and jacobian at https://github.com/JuliaDiff/ForwardDiff.jl/tree/master/test

The work on wrapping API of ForwardDiff can be thought as done, with further things considered as big improvements:

• Wrapping mutating API of ForwardDiff like derivative!.
• More detailed documentation, with more examples preferably.
• Adaptation of tests in https://github.com/JuliaDiff/ForwardDiff.jl/blob/master/test/MiscTest.jl and https://github.com/JuliaDiff/ForwardDiff.jl/blob/master/test/ConfusionTest.jl