Having a jvp function for jacobian vector product

[theogf]

Following a discussion on Slack we realized there is no Jacobian-Vector product function in ForwardDiff.jl.
The idea would be to have a function jvp(foo, x, v) = jacobian(foo, x) * v.
A solution proposed is to rewrite it as a derivative:

function jvp(foo, x, v)
  derivative(0) do t
    foo(x .+ t .* v)
  end
end

Edit : Same issue as #319

[KristofferC]

Close as dup